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 hwclock(8) man page.
The format of the adjtime file is, in ASCII.
- 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 floating point decimal
- 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.
- clock mode
- Supported values are "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.