other options:
--utc --directisa --test --debug
Minimum unique abbreviations of all options are acceptable.
Also, equivalent options -r, -w, -s, -a, -v, -u, and -D are accepted for compatibility with the program "clock".
You can also run hwclock periodically to insert or remove time from the Hardware Clock to compensate for systematic drift (where the clock consistently gains or loses time at a certain rate if left to run).
This epoch value is used whenever hwclock reads or sets the Hardware Clock.
hwclock --set --date=9/22/96 16:45:05
For example,
hwclock --setepoch --epoch=1952
The following options apply to most functions.
If you don't specify --utc when you should, or vice versa, both setting and querying of the Hardware Clock will be messed up.
The rtc device driver was new in Linux Release 2.
There are two main clocks in a Linux system:
The Hardware Clock: This is a clock that runs independently of any control program running in the CPU and even when the machine is powered off.
On an ISA system, this clock is specified as part of the ISA standard. The control program can read or set this clock to a whole second, but the control program 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 because all of the other names are inappropriate to the point of being misleading.
The System Time: This is the time kept by a clock inside the Linux kernel and 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). 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 in a Linux system is to keep time when Linux is not running. You initialize the System Time to the time from the Hardware Clock when Linux starts up, and then never use the Hardware Clock again. 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(1L) 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. You can also use the program adjtimex(8) to smoothly adjust the System Time while the system runs.
hwclock Uses many different ways to get and set Hardware Clock values. The most normal way is to do I/O to the device special file /dev/rtc, which is presumed to be driven by the rtc device driver. However, this method is not always available. For one thing, the rtc driver is a relatively recent addition to Linux. Older systems don't have it.
On older systems, the method of accessing the Hardware Clock depends on the system hardware.
On an ISA system, hwclock can directly access the "CMOS memory" registers that constitute the clock, by doing I/O to Ports 0x70 and 0x71. It can only do this if running with superuser effective userid.
This is a really poor method of accessing the clock, for all the reasons that user space programs are generally not supposed to do direct I/O and disable interrupts. Hwclock provides it because it is the only method available with older Linux kernels for ISA machines.
On an m68k system, hwclock can access the clock via the console driver, via the device special file /dev/tty1.
On an Alpha, /dev/rtc is the only choice.
There are or were some Alpha Linux systems that don't have /dev/rtc and there are or were programs that accessed the clock via almost direct I/O using /dev/port. However, this is not as good a method as /dev/rtc and such programs were not widely enough used that hwclock has any need to be backward compatible with them. So hwclock does not provide the /dev/port method and consequently will not work on an Alpha that doesn't have /dev/rtc.
hwclock tries to use /dev/rtc. If it is compiled for a kernel that doesn't have that function or it is unable to open /dev/rtc, hwclock will fall back to another method, if available. On an ISA machine, you can force hwclock to use the direct manipulation of the CMOS registers without even trying /dev/rtc by specifying the --directisa option.
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 make systematic corrections to correct the systematic drift.
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. 5 days later, the clock has gained 10 seconds, so you issue another hwclock --set 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 goes 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.
Every time you calibrate (set) the clock, hwclock recalculates the systematic drift rate based on how long it has been since the last calibration, how long it has been since the last adjustment, what drift rate was assumed in any intervening adjustments, and the amount by which the clock is presently off.
A small amount of error creeps in any time hwclock sets the clock, so it refrains from making an adjustment that would be less than 1 second. Later on, when you request an adjustment again, the accumulated drift will be more than a second and hwclock will do the adjustment then.
It is good to do a hwclock --adjust just before the hwclock --hctosys at system startup time, and maybe periodically while the system is running via cron.
The format of the adjtime file is:
Line 1: 3 numbers: 1) systematic drift rate in seconds per day, floating point decimal; 2) Resulting number of seconds since 1969 UTC of most recent adjustment or calibration, decimal integer; 3) zero (for compatibility with clock ).
Line 2: 1 number: Resulting number of seconds since 1969 UTC of most recent calibration.
You can use an adjtime file that was previously used with the clock program with hwclock.