The NTP driftfile

The driftfile entry in /etc/ntp.conf tells ntpd 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 ntpd's internal value of the frequency error. The file is updated once every hour by ntpd. 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 ntpd needs a good estimate to synchronize closely to its server, there should always be a driftfile entry in /etc/ntp.conf.