The NTP driftfile
The driftfile entry in /etc/ntp.conf
tells xntpd the name of the file
where it can find and store the clock drift, also known as
frequency error, of the system clock.
If the file exists at startup, it is read and the value is used to
initialize xntpd's internal value of the frequency error.
The file is updated once every hour by xntpd.
It usually takes a day or so after the daemon is started to compute
a good estimate of the clock drift.
Once the initial value is computed, it will change only by relatively
small amounts during the course of continued operation.
Because xntpd needs a good estimate to synchronize
closely to its server,
there should always be a driftfile entry in
/etc/ntp.conf.