Section (5) adjtime_config
adjtime — information about hardware clock setting and drift factor
contains descriptive information about the hardware mode
clock setting and clock drift factor. The file is read and
write by hwclock; and read by programs like rtcwake to get
RTC time mode.
The file is usually located in /etc, but tools like hwclock(8) or rtcwake(8) allow to use alternative location by command line options if write access to /etc is unwanted. The default clock mode is UTC if the file is missing.
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. The util hwclock keeps the file /etc/adjtime, that keeps some historical information. For more details see The Adjust Function and The Adjtime File sections from hwckock(8) man page.
The format of the adjtime file is, in ASCII.
Three numbers, separated by blanks:
- drift factor
the systematic drift rate in seconds per day (floating point decimal)
- last adjust time
the resulting number of seconds since 1969 UTC of most recent adjustment or calibration (decimal integer)
- adjustment status
zero (for compatibility with clock(8)) as a decimal integer.
- last calibration time
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.