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HWCLOCK

Section: System Administration (8)
Updated: January 2015
Index Return to Main Contents
 

NAME

hwclock - read or set the hardware clock (RTC)  

SYNOPSIS

hwclock [function] [option...]  

DESCRIPTION

hwclock is a tool for accessing the Hardware Clock. It can: display the Hardware Clock time; set the Hardware Clock to a specified time; set the Hardware Clock from the System Clock; set the System Clock from the Hardware Clock; compensate for Hardware Clock drift; correct the System Clock timescale; set the kernel's timezone, NTP timescale, and epoch (Alpha only); compare the System and Hardware Clocks; and predict future Hardware Clock values based on its drift rate.

Since v2.26 important changes were made to the --hctosys function, the --directisa option, and a new option --update-drift was added. See their respective sections below.  

FUNCTIONS

The following functions are mutually exclusive, only one can be given at a time. If none are given the default is --show.
--adjust
Add or subtract time from the Hardware Clock to account for systematic drift since the last time the clock was set or adjusted. See the discussion below, under The Adjust Function.
-c, --compare
Periodically compare the Hardware Clock to the System Time and output the difference every 10 seconds. This will also print the frequency offset and tick.
--getepoch

--setepoch
These functions are for Alpha machines only.

Read and set the kernel's Hardware Clock epoch value. Epoch is the number of years into AD to which a zero year value in the Hardware Clock refers. For example, if you are using the convention that the year counter in your Hardware Clock contains the number of full years since 1952, then the kernel's Hardware Clock epoch value must be 1952.

The set function requires using the --epoch option.

This epoch value is used whenever hwclock reads or sets the Hardware Clock.

--predict
Predict what the Hardware Clock will read in the future based upon the time given by the --date option and the information in /etc/adjtime. This is useful, for example, to account for drift when setting a Hardware Clock wakeup (aka alarm). See rtcwake(8).

Do not use this function if the Hardware Clock is being modified by anything other than the current operating system's hwclock command, such as '11 minute mode' or from dual-booting another OS.

-r, --show

--get
Read the Hardware Clock and print the time on standard output. The time shown is always in local time, even if you keep your Hardware Clock in UTC. See the --localtime option.

Showing the Hardware Clock time is the default when no function is specified.

The --get function also applies drift correction to the time read, based upon the information in /etc/adjtime. Do not use this function if the Hardware Clock is being modified by anything other than the current operating system's hwclock command, such as '11 minute mode' or from dual-booting another OS.

-s, --hctosys
Set the System Clock from the Hardware Clock. The time read from the Hardware Clock is compensated to account for systematic drift before using it to set the System Clock. See the discussion below, under The Adjust Function.

The System Clock must be kept in the UTC timescale for date-time applications to work correctly in conjunction with the timezone configured for the system. If the Hardware Clock is kept in local time then the time read from it must be shifted to the UTC timescale before using it to set the System Clock. The --hctosys function does this based upon the information in the /etc/adjtime file or the command line arguments --localtime and --utc. Note: no daylight saving adjustment is made. See the discussion below under LOCAL vs UTC.

The kernel also keeps a timezone value, the --hctosys function sets it to the timezone configured for the system. The system timezone is configured by the TZ environment variable or the /etc/localtime file, as tzset(3) would interpret them. The obsolete tz_dsttime field of the kernel's timezone value is set to zero. (For details on what this field used to mean, see settimeofday(2).)

When used in a startup script, making the --hctosys function the first caller of settimeofday(2) from boot, it will set the NTP '11 minute mode' timescale via the persistent_clock_is_local kernel variable. If the Hardware Clock's timescale configuration is changed then a reboot is required to inform the kernel. See the discussion below, under Automatic Hardware Clock Synchronization by the Kernel.

This is a good function to use in one of the system startup scripts before the file systems are mounted read/write.

This function should never be used on a running system. Jumping system time will cause problems, such as, corrupted filesystem timestamps. Also, if something has changed the Hardware Clock, like NTP's '11 minute mode', then --hctosys will set the time incorrectly by including drift compensation.

Drift compensation can be inhibited by setting the drift factor in /etc/adjtime to zero. This setting will be persistent as long as the --update-drift option is not used with --systohc at shutdown (or anywhere else). Another way to inhibit this is by using the --noadjfile option when calling the --hctosys function. A third method is to delete the /etc/adjtime file. Hwclock will then default to using the UTC timescale for the Hardware Clock. If the Hardware Clock is ticking local time it will need to be defined in the file. This can be done by calling hwclock --localtime --adjust; when the file is not present this command will not actually adjust the Clock, but it will create the file with local time configured, and a drift factor of zero.

A condition under which inhibiting hwclock's drift correction may be desired is when dual-booting multiple operating systems. If while this instance of Linux is stopped, another OS changes the Hardware Clock's value, then when this instance is started again the drift correction applied will be incorrect.

For hwclock's drift correction to work properly it is imperative that nothing changes the Hardware Clock while its Linux instance is not running.

--set
Set the Hardware Clock to the time given by the --date option, and update the timestamps in /etc/adjtime. With the --update-drift option (re)calculate the drift factor.
--systz
This is an alternate to the --hctosys function that does not read the Hardware Clock nor set the System Clock; consequently there is not any drift correction. It is intended to be used in a startup script on systems with kernels above version 2.6 where you know the System Clock has been set from the Hardware Clock by the kernel during boot.

It does the following things that are detailed above in the --hctosys function:

*
Corrects the System Clock timescale to UTC as needed. Only instead of accomplishing this by setting the System Clock, hwclock simply informs the kernel and it handles the change.
*
Sets the kernel's NTP '11 minute mode' timescale.
*
Sets the kernel's timezone.

The first two are only available on the first call of settimeofday(2) after boot. Consequently this option only makes sense when used in a startup script. If the Hardware Clocks timescale configuration is changed then a reboot would be required to inform the kernel.

-w, --systohc
Set the Hardware Clock from the System Clock, and update the timestamps in /etc/adjtime. With the --update-drift option (re)calculate the drift factor.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
 

OPTIONS

--adjfile=filename
Override the default /etc/adjtime file path.
--badyear
Indicate that the Hardware Clock is incapable of storing years outside the range 1994-1999. There is a problem in some BIOSes (almost all Award BIOSes made between 4/26/94 and 5/31/95) wherein they are unable to deal with years after 1999. If one attempts to set the year-of-century value to something less than 94 (or 95 in some cases), the value that actually gets set is 94 (or 95). Thus, if you have one of these machines, hwclock cannot set the year after 1999 and cannot use the value of the clock as the true time in the normal way.

To compensate for this (without your getting a BIOS update, which would definitely be preferable), always use --badyear if you have one of these machines. When hwclock knows it's working with a brain-damaged clock, it ignores the year part of the Hardware Clock value and instead tries to guess the year based on the last calibrated date in the adjtime file, by assuming that date is within the past year. For this to work, you had better do a hwclock --set or hwclock --systohc at least once a year!

Though hwclock ignores the year value when it reads the Hardware Clock, it sets the year value when it sets the clock. It sets it to 1995, 1996, 1997, or 1998, whichever one has the same position in the leap year cycle as the true year. That way, the Hardware Clock inserts leap days where they belong. Again, if you let the Hardware Clock run for more than a year without setting it, this scheme could be defeated and you could end up losing a day.

hwclock warns you that you probably need --badyear whenever it finds your Hardware Clock set to 1994 or 1995.

--date=date_string
You need this option if you specify the --set or --predict functions, otherwise it is ignored. It specifies the time to which to set the Hardware Clock, or the time for which to predict the Hardware Clock reading. The value of this option is used as an argument to the date(1) program's --date option. For example:
hwclock --set --date='2011-08-14 16:45:05'

The argument must be in local time, even if you keep your Hardware Clock in UTC. See the --localtime option. The argument must not be a relative time like "+5 minutes", because hwclock's precision depends upon correlation between the argument's value and when the enter key is pressed.

--debug
Display a lot of information about what hwclock is doing internally. Some of its functions are complex and this output can help you understand how the program works.
--directisa
This option is meaningful for: ISA compatible machines including x86, and x86_64; and Alpha (which has a similar Hardware Clock interface). For other machines, it has no effect. This option tells hwclock to use explicit I/O instructions to access the Hardware Clock. Without this option, hwclock will use the rtc device, which it assumes to be driven by the RTC device driver. As of v2.26 it will no longer automatically use directisa when the rtc driver is unavailable; this was causing an unsafe condition that could allow two processes to access the Hardware Clock at the same time. Direct hardware access from userspace should only be used for testing, troubleshooting, and as a last resort when all other methods fail. See the --rtc option.
-f, --rtc=filename
Override hwclock's default rtc device file name. Otherwise it will use the first one found in this order:
/dev/rtc
/dev/rtc0
/dev/misc/rtc
For IA-64:
/dev/efirtc
/dev/misc/efirtc
--localtime

-u, --utc
Indicate which timescale the Hardware Clock is set to.

The Hardware Clock may be configured to use either the UTC or the local timescale, but nothing in the clock itself says which alternative is being used. The --localtime or --utc options give this information to the hwclock command. If you specify the wrong one (or specify neither and take a wrong default), both setting and reading the Hardware Clock will be incorrect.

If you specify neither --utc nor --localtime then the one last given with a set function (--set, --systohc, or --adjust), as recorded in /etc/adjtime, will be used. If the adjtime file doesn't exist, the default is UTC.

Note: daylight saving time changes may be inconsistent when the Hardware Clock is kept in local time. See the discussion below under LOCAL vs UTC.

--noadjfile
Disable the facilities provided by /etc/adjtime. hwclock will not read nor write to that file with this option. Either --utc or --localtime must be specified when using this option.
--test
Do not actually change anything on the system, i.e., the Clocks or adjtime file. This is useful, especially in conjunction with --debug, in learning about the internal operations of hwclock.
--update-drift
Update the Hardware Clock's drift factor in /etc/adjtime. It is used with --set or --systohc, otherwise it is ignored.

A minimum four hour period between settings is required. This is to avoid invalid calculations. The longer the period, the more precise the resulting drift factor will be.

This option was added in v2.26, because it is typical for systems to call hwclock --systohc at shutdown; with the old behaviour this would automatically (re)calculate the drift factor which caused several problems:

*
When using ntpd with an '11 minute mode' kernel the drift factor would be clobbered to near zero.
*
It would not allow the use of 'cold' drift correction. With most configurations using 'cold' drift will yield favorable results. Cold, means when the machine is turned off which can have a significant impact on the drift factor.
*
(Re)calculating drift factor on every shutdown delivers suboptimal results. For example, if ephemeral conditions cause the machine to be abnormally hot the drift factor calculation would be out of range.

Having hwclock calculate the drift factor is a good starting point, but for optimal results it will likely need to be adjusted by directly editing the /etc/adjtime file. For most configurations once a machine's optimal drift factor is crafted it should not need to be changed. Therefore, the old behavior to automatically (re)calculate drift was changed and now requires this option to be used. See the discussion below, under The Adjust Function.

 

OPTIONS FOR ALPHA MACHINES ONLY

--arc
This option is equivalent to --epoch=1980 and is used to specify the most common epoch on Alphas with ARC console (but Ruffians have an epoch of 1900).
--epoch=year
Specifies the year which is the beginning of the Hardware Clock's epoch, that is the number of years into AD to which a zero value in the Hardware Clock's year counter refers. It is used together with the --setepoch option to set the kernel's idea of the epoch of the Hardware Clock.

For example, on a Digital Unix machine:

hwclock --setepoch --epoch=1952
--funky-toy

--jensen
These two options specify what kind of Alpha machine you have. They are invalid if you do not have an Alpha and are usually unnecessary if you do; hwclock should be able to determine what it is running on when /proc is mounted.

The --jensen option is used for Jensen models; --funky-toy means that the machine requires the UF bit instead of the UIP bit in the Hardware Clock to detect a time transition. "Toy" in the option name refers to the Time Of Year facility of the machine.

--srm
This option is equivalent to --epoch=1900 and is used to specify the most common epoch on Alphas with SRM console.
 

NOTES

 

Clocks in a Linux System

There are two types of date-time clocks:

The Hardware Clock: This clock is an independent hardware device, with its own power domain (battery, capacitor, etc), that operates when the machine is powered off, or even unplugged.

On an ISA compatible system, this clock is specified as part of the ISA standard. A control program can read or set this clock only to a whole second, but it can also detect the edges of the 1 second clock ticks, so the clock actually has virtually infinite precision.

This clock is commonly called the hardware clock, the real time clock, the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its capitalized form, was coined for use by hwclock. The Linux kernel also refers to it as the persistent clock.

Some non-ISA systems have a few real time clocks with only one of them having its own power domain. A very low power external I2C or SPI clock chip might be used with a backup battery as the hardware clock to initialize a more functional integrated real-time clock which is used for most other purposes.

The System Clock: This clock is part of the Linux kernel and is driven by a timer interrupt. (On an ISA machine, the timer interrupt is part of the ISA standard.) It has meaning only while Linux is running on the machine. The System Time is the number of seconds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number of seconds since 1969 UTC). The System Time is not an integer, though. It has virtually infinite precision.

The System Time is the time that matters. The Hardware Clock's basic purpose is to keep time when Linux is not running so that the System Clock can be initialized from it at boot. Note that in DOS, for which ISA was designed, the Hardware Clock is the only real time clock.

It is important that the System Time not have any discontinuities such as would happen if you used the date(1) program to set it while the system is running. You can, however, do whatever you want to the Hardware Clock while the system is running, and the next time Linux starts up, it will do so with the adjusted time from the Hardware Clock. Note: currently this is not possible on most systems because hwclock --systohc is called at shutdown.

The Linux kernel's timezone is set by hwclock. But don't be misled -- almost nobody cares what timezone the kernel thinks it is in. Instead, programs that care about the timezone (perhaps because they want to display a local time for you) almost always use a more traditional method of determining the timezone: They use the TZ environment variable or the /etc/localtime file, as explained in the man page for tzset(3). However, some programs and fringe parts of the Linux kernel such as filesystems use the kernel's timezone value. An example is the vfat filesystem. If the kernel timezone value is wrong, the vfat filesystem will report and set the wrong timestamps on files. Another example is the kernel's NTP '11 minute mode.' If the kernel's timezone value and/or the persistent_clock_is_local variable are wrong, then the Hardware Clock will be set incorrectly by '11 minute mode.' See the discussion below, under Automatic Hardware Clock Synchronization by the Kernel.

hwclock sets the kernel's timezone to the value indicated by TZ or /etc/localtime with the --hctosys or --systz functions.

The kernel's timezone value actually consists of two parts: 1) a field tz_minuteswest indicating how many minutes local time (not adjusted for DST) lags behind UTC, and 2) a field tz_dsttime indicating the type of Daylight Savings Time (DST) convention that is in effect in the locality at the present time. This second field is not used under Linux and is always zero. See also settimeofday(2).  

User access and setuid

Sometimes, you need to install hwclock setuid root. If you want users other than the superuser to be able to display the clock value using the direct ISA I/O method, install it setuid root. If you have the rtc device interface on your system, or are on a non-ISA compatible system, there is probably no need for users to have the direct ISA I/O method, so do not bother. See the --rtc option.

In any case, hwclock will not allow you to set anything unless you have the superuser real uid. (This restriction is not necessary if you haven't installed setuid root, but it's there for now.)  

Hardware Clock Access Methods

hwclock uses many different ways to get and set Hardware Clock values. The most normal way is to do I/O to the rtc device special file, which is presumed to be driven by the rtc device driver. Also, Linux systems using the rtc framework with udev, are capable of supporting multiple Hardware Clocks. This may bring about the need to override the default rtc device by specifying one with the --rtc option.

However, this method is not always available as older systems do not have an rtc driver. On these systems, the method of accessing the Hardware Clock depends on the system hardware.

On an ISA compatible system, hwclock can directly access the "CMOS memory" registers that constitute the clock, by doing I/O to Ports 0x70 and 0x71. It does this with actual I/O instructions and consequently can only do it if running with superuser effective userid. This method may be used by specifying the --directisa option.

This is a really poor method of accessing the clock, for all the reasons that userspace programs are generally not supposed to do direct I/O and disable interrupts. hwclock provides it for testing, troubleshooting, and because it may be the only method available on ISA compatible and Alpha systems which do not have a working rtc device driver.

In the case of a Jensen Alpha, there is no way for hwclock to execute those I/O instructions, and so it uses instead the /dev/port device special file, which provides almost as low-level an interface to the I/O subsystem.

On an m68k system, hwclock can access the clock with the console driver, via the device special file /dev/tty1.  

The Adjust Function

The Hardware Clock is usually not very accurate. However, much of its inaccuracy is completely predictable - it gains or loses the same amount of time every day. This is called systematic drift. hwclock's --adjust function lets you apply systematic drift corrections to the Hardware Clock.

It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical information. This is called the adjtime file.

Suppose you start with no adjtime file. You issue a hwclock --set command to set the Hardware Clock to the true current time. hwclock creates the adjtime file and records in it the current time as the last time the clock was calibrated. Five days later, the clock has gained 10 seconds, so you issue a hwclock --set --update-drift command to set it back 10 seconds. hwclock updates the adjtime file to show the current time as the last time the clock was calibrated, and records 2 seconds per day as the systematic drift rate. 24 hours go by, and then you issue a hwclock --adjust command. hwclock consults the adjtime file and sees that the clock gains 2 seconds per day when left alone and that it has been left alone for exactly one day. So it subtracts 2 seconds from the Hardware Clock. It then records the current time as the last time the clock was adjusted. Another 24 hours go by and you issue another hwclock --adjust. hwclock does the same thing: subtracts 2 seconds and updates the adjtime file with the current time as the last time the clock was adjusted.

When you use the --update-drift option with --set or --systohc, the systematic drift rate is (re)calculated by comparing the fully drift corrected current Hardware Clock time with the new set time, from that it derives the 24 hour drift rate based on the last calibrated timestamp from the adjtime file. This updated drift factor is then saved in /etc/adjtime.

A small amount of error creeps in when the Hardware Clock is set, so --adjust refrains from making any adjustment that is less than 1 second. Later on, when you request an adjustment again, the accumulated drift will be more than 1 second and --adjust will make the adjustment including any fractional amount.

hwclock --hctosys also uses the adjtime file data to compensate the value read from the Hardware Clock before using it to set the System Clock. It does not share the 1 second limitation of --adjust, and will correct sub-second drift values immediately. It does not change the Hardware Clock time nor the adjtime file. This may eliminate the need to use --adjust, unless something else on the system needs the Hardware Clock to be compensated.  

The Adjtime File

While named for its historical purpose of controlling adjustments only, it actually contains other information used by hwclock from one invocation to the next.

The format of the adjtime file is, in ASCII:

Line 1: Three numbers, separated by blanks: 1) the systematic drift rate in seconds per day, floating point decimal; 2) the resulting number of seconds since 1969 UTC of most recent adjustment or calibration, decimal integer; 3) zero (for compatibility with clock(8)) as a decimal integer.

Line 2: One number: the resulting number of seconds since 1969 UTC of most recent calibration. Zero if there has been no calibration yet or it is known that any previous calibration is moot (for example, because the Hardware Clock has been found, since that calibration, not to contain a valid time). This is a decimal integer.

Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to Coordinated Universal Time or local time. You can always override this value with options on the hwclock command line.

You can use an adjtime file that was previously used with the clock(8) program with hwclock.  

Automatic Hardware Clock Synchronization by the Kernel

You should be aware of another way that the Hardware Clock is kept synchronized in some systems. The Linux kernel has a mode wherein it copies the System Time to the Hardware Clock every 11 minutes. This is a good mode to use when you are using something sophisticated like NTP to keep your System Clock synchronized. (NTP is a way to keep your System Time synchronized either to a time server somewhere on the network or to a radio clock hooked up to your system. See RFC 1305.)

This mode (we'll call it '11 minute mode') is off until something turns it on. The NTP daemon ntpd is one thing that turns it on. You can turn it off by running anything, including hwclock --hctosys, that sets the System Clock the old fashioned way. However, if the NTP daemon is still running, it will turn 11 minute mode back on again the next time it synchronizes the System Clock.

When '11 minute mode' is active the 64 bit of the kernel's time_status variable is unset. The status variable can be checked with the adjtimex --print or ntptime commands.

If your system runs with '11 minute mode' on, it may need to use either --hctosys or --systz in a startup script, especially if the Hardware Clock is configured to use the local timescale. Unless the kernel is informed of what timescale the Hardware Clock is using, it may clobber it with the wrong one. The kernel uses UTC by default.

The first userspace command to set the System Clock informs the kernel what timescale the Hardware Clock is using. This happens via the persistent_clock_is_local kernel variable. If --hctosys or --systz is the first, it will set this variable according to the adjtime file or the appropriate command-line argument. Note that when using this capability and the Hardware Clock timescale configuration is changed, then a reboot is required to notify the kernel.

hwclock --adjust should not be used with NTP '11 minute mode.'  

ISA Hardware Clock Century value

There is some sort of standard that defines CMOS memory Byte 50 on an ISA machine as an indicator of what century it is. hwclock does not use or set that byte because there are some machines that don't define the byte that way, and it really isn't necessary anyway, since the year-of-century does a good job of implying which century it is.

If you have a bona fide use for a CMOS century byte, contact the hwclock maintainer; an option may be appropriate.

Note that this section is only relevant when you are using the "direct ISA" method of accessing the Hardware Clock. ACPI provides a standard way to access century values, when they are supported by the hardware.  

DATE-TIME CONFIGURATION

 

Keeping Time without External Synchronization

This discussion is based on the following conditions:

*
Nothing is running that alters the date-time clocks, e.g., ntpd(1), cron jobs, et al.
*
The system timezone is configured for the correct local time. See below POSIX vs 'RIGHT'.
*
Early in startup the following are called in this order:
adjtimex --tick <value> --frequency <value>
hwclock --hctosys
*
During shutdown the following is called:
hwclock --systohc

* Systems without adjtimex may use ntptime.

Whether maintaining precision time with ntpd(1) or not, it makes sense to configure the system to keep reasonably good date-time on its own.

The first step in making that happen is having a clear understanding of the big picture. There are two completely separate hardware devices running at their own speed and drifting away from the 'correct' time at their own rates. The methods and software for drift correction are different for each of them. However, most systems are configured to exchange values between these two clocks at startup and shutdown. Now the individual device's time keeping errors are transferred back and forth between each other. Attempt to configure drift correction for only one of them, and the other's drift will be overlaid upon it. If the big picture is not kept in mind, confusion will soon ensue.

This problem can be avoided when configuring drift correction for the System Clock by simply not shutting down the machine. This, plus the fact that all of hwclock's precision (including calculating drift factors) depends upon the System Clock's rate being correct, means that configuration of the System Clock should be done first.

The System Clock drift is corrected with the adjtimex(8) command's --tick and --frequency options. These two work together, tick is the course adjustment and frequency is the fine adjustment. (For system that do not have an adjtimex package, ntptime -f <ppm> may be use instead.)

Some Linux distributions attempt to automatically calculate the System Clock drift with adjtimex's compare operation. Trying to correct one drifting clock by using another drifting clock as a reference is akin to a dog trying to catch its own tail. Success may happen eventually, but great effort and frustration will likely precede it. This automation may yield an improvement over no configuration, but expecting optimum results would be in error. A better choice for manual configuration would be adjtimex's --log options.

It may be more effective to simply track the System Clock drift with ntpdate -q , or date -Ins and a precision timepiece, and then calculate the correction manually.

After setting the tick and frequency values, continue to test and refine the adjustments until the System Clock keeps good time. See adjtimex(8) for more information and the example demonstrating manual drift calculations.

Once the System Clock is ticking smoothly, move on to the Hardware Clock.

As a rule, cold drift will work best for most use cases. This should be true even for 24/7 machines whose normal downtime consists of a reboot. In that case the drift factor value makes little difference, but on the rare occasion that the machine is shutdown for an extended period then cold drift should yield better results.

Steps to calculate cold drift:

1
Confirm that ntpd(1) will not be launched at startup.
2
The System Clock time must be correct at shutdown!
3
Shutdown the system.
4
Let an extended period pass without changing the Hardware Clock.
5
Start the system.
6
Immediately use hwclock to set the correct time with the --update-drift option.

Note: if step six uses --systohc, then the System Clock must be set correctly (step 6a) just before doing so.

Having hwclock calculate the drift factor is a good starting point, but for optimal results it will likely need to be adjusted by directly editing the /etc/adjtime file. Continue to test and refine the drift factor until the Hardware Clock is corrected properly at startup. To check this, first make sure that the System Time is correct before shutdown and then use ntpdate -q, or date -Ins and a precision timepiece, immediately after startup.

Both clocks typically use a quartz crystal oscillator. Crystals are used for reference oscillators in electronics because by most measures they produce a very clean and stable sine wave. Their greatest shortcoming is that they have a Positive Temperature Coefficient; meaning that their frequency increases as the temperature increases and vise versa. Therefore, both the Hardware and System Clock's drift rate changes with intrinsic and extrinsic machine temperatures. These characteristics will vary by machine depending upon its design.

Drift correction strategies are many, but as a general guide the goal would be to find a longterm average. A year long average to take into account seasonal ambient temperature shifts may be a good target period. So perhaps the date-time advances a bit in the summer and declines a bit in the winter, but at the end of a year it balances to zero.

If this is beginning to sound futile, it is not. Left on its own a machine can lose 3 seconds per day or more. Accumulated drift over a year may easily exceed half an hour. Using carefully crafted drift corrections can make a significant improvement in a machine's ability to keep reasonably good date-time.  

LOCAL vs UTC

Keeping the Hardware Clock in a local timescale causes inconsistent daylight saving time results:
*
If Linux is running during a daylight saving time change, the time written to the Hardware Clock will be adjusted for the change.
*
If Linux is NOT running during a daylight saving time change, the time read from the Hardware Clock will NOT be adjusted for the change.

The Hardware Clock on an ISA compatible system keeps only a date and time, it has no concept of timezone nor daylight saving. Therefore, when hwclock is told that it is in local time, it assumes it is in the 'correct' local time and makes no adjustments to the time read from it.

Linux handles daylight saving time changes transparently only when the Hardware Clock is kept in the UTC timescale. Doing so is made easy for system administrators as hwclock uses local time for its output and as the argument to the --date option.

POSIX systems, like Linux, are designed to have the System Clock operate in the UTC timescale. The Hardware Clock's purpose is to initialize the System Clock, so also keeping it in UTC makes sense.

Linux does, however, attempt to accommodate the Hardware Clock being in the local timescale. This is primarily for dual-booting with older versions of MS Windows. From Windows 7 on, the RealTimeIsUniversal registry key is supposed to be working properly so that its Hardware Clock can be kept in UTC.  

POSIX vs 'RIGHT'

A discussion on date-time configuration would be incomplete without addressing timezones, this is mostly well covered by tzset(3). One area that seems to have no documentation is the 'right' directory of the Time Zone Database, sometimes called tz or zoneinfo.

There are two separate databases in the zoneinfo system, posix and 'right'. 'Right' (now named zoneinfo-leaps) includes leap seconds and posix does not. To use the 'right' database the System Clock must be set to (UTC + leap seconds), which is equivalent to (TAI - 10). This allows calculating the exact number of seconds between two dates that cross a leap second epoch. The System Clock is then converted to the correct civil time, including UTC, by using the 'right' timezone files which subtract the leap seconds. Note: this configuration is considered experimental and is known to have issues.

To configure a system to use a particular database all of the files located in its directory must be copied to the root of /usr/share/zoneinfo. Files are never used directly from the posix or 'right' subdirectories, e.g., TZ='right/Europe/Dublin'. This habit was becoming so common that the upstream zoneinfo project restructured the system's file tree by moving the posix and 'right' subdirectories out of the zoneinfo directory and into sibling directories:

/usr/share/zoneinfo
/usr/share/zoneinfo-posix
/usr/share/zoneinfo-leaps

Unfortunately, some Linux distributions are changing it back to the old tree structure in their packages. So the problem of system administrators reaching into the 'right' subdirectory persists. This causes the system timezone to be configured to include leap seconds while the zoneinfo database is still configured to exclude them. Then when an application such as a World Clock needs the South_Pole timezone file; or an email MTA, or hwclock needs the UTC timezone file; they fetch it from the root of /usr/share/zoneinfo , because that is what they are supposed to do. Those files exclude leap seconds, but the System Clock now includes them, causing an incorrect time conversion.

Attempting to mix and match files from these separate databases will not work, because they each require the System Clock to use a different timescale. The zoneinfo database must be configured to use either posix or 'right', as described above.  

ENVIRONMENT VARIABLES

TZ  

FILES


/etc/adjtime
/etc/localtime
/dev/rtc
/dev/rtc0
/dev/misc/rtc
/dev/efirtc
/dev/misc/efirtc
/dev/port
/dev/tty1
/proc/cpuinfo  

SEE ALSO

date(1), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)  

AUTHORS

Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com), based on work done on the clock(8) program by Charles Hedrick, Rob Hooft, and Harald Koenig. See the source code for complete history and credits.  

AVAILABILITY

The hwclock command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
FUNCTIONS
OPTIONS
OPTIONS FOR ALPHA MACHINES ONLY
NOTES
Clocks in a Linux System
User access and setuid
Hardware Clock Access Methods
The Adjust Function
The Adjtime File
Automatic Hardware Clock Synchronization by the Kernel
ISA Hardware Clock Century value
DATE-TIME CONFIGURATION
Keeping Time without External Synchronization
LOCAL vs UTC
POSIX vs 'RIGHT'
ENVIRONMENT VARIABLES
FILES
SEE ALSO
AUTHORS
AVAILABILITY


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