Chapter 4 · Essential System Services·v1.0.0·Updated 7/6/2026·~11 min
What's changed: Initial version (topic 1.09, subtopics 1.09.1–1.09.3)
4.1Managing System Time
Key points
Learn to keep and synchronize system time: date and the hardware clock (hwclock), time zone configuration (/usr/share/zoneinfo/, /etc/localtime, /etc/timezone, TZ, tzselect, timedatectl), and NTP synchronization (ntpd, ntpdate, ntpq, chronyd, chronyc, pool.ntp.org).
Correlating logs, validating certificates, authentication—accurate time underpins everything. Linux keeps two clocks (the OS-maintained system clock and the motherboard's hardware clock), and the standard practice aligns them to internet time sources via NTP.
4.1.1The two clocks and time zones
- date shows/sets the system clock (
date, formatteddate +%Y-%m-%d). hwclock reads/syncs the hardware clock (hwclock --systohcwrites system→hardware;--hctosysthe reverse). - Time zones live as files under /usr/share/zoneinfo/; configuration means symlinking /etc/localtime to one (Debian family also records the name in /etc/timezone).
- Set with timedatectl (the systemd-era integrated tool:
timedatectl set-timezone Asia/Tokyo, plus status), pick interactively with tzselect, or switch temporarily via the TZ variable (TZ=UTC date).
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