Part I · How it works

10. How time works

1. 10.1Wall clock and monotonic clock: measuring time rightThe wall clock says what time it is. It can jump: NTP corrects it, an admin sets it, a VM resumes. Never measure durations with it.
2. 10.2Keeping the clock right: NTPComputer clocks drift, by seconds a day. NTP asks time servers and corrects the clock continuously, to within milliseconds over the Internet.
3. 10.3Time zones, UTC and daylight savingThe kernel counts time in UTC. Time zones are applied when time is displayed, by the C library, from the rules in /etc/localtime.
4. 10.4Timestamps: Unix time, 2038 and leap secondsUnix time counts seconds since 1970-01-01 00:00 UTC: one number, the same everywhere, with no time zone. Most systems store time this way.