What's changed: Initial version (topic 1.09, subtopics 1.09.1–1.09.3)
4.1Managing System Time
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).
4.1.2Synchronizing with NTP
- NTP synchronizes with time sources over the network (UDP/123). The standard public source is pool.ntp.org—a worldwide server pool served via DNS.
- Traditional: ntpd (daemon, gradual correction; config in ntp.conf), ntpdate (one-shot immediate set; deprecated), ntpq (
ntpq -pshows peers and sync state). - Current mainstream: chronyd (the successor daemon, robust on flaky links and VMs; config in chrony.conf) with chronyc (
chronyc sourceslists references).
Staples: hwclock --systohc writes system→hardware (direction questions), set zones with timedatectl set-timezone (implemented as the /etc/localtime link), gradual daemon correction = ntpd/chronyd vs one-shot = ntpdate, check sync with ntpq -p / chronyc sources. NTP's UDP/123 also reappears among the 1.07 port questions.
Frame the two clocks operationally. At power-on the OS reads the battery-backed hardware clock to initialize the system clock; thereafter the OS ticks the system clock, and NTP daemons correct that side. Hence the tradition of writing back with hwclock --systohc before shutdown or after large corrections (automated under systemd). Why do ntpd/chronyd correct gradually (slew)? Because time going backwards reorders logs, skips or duplicates cron runs, and can corrupt databases. That is why one-shot tools like ntpdate are dangerous on live servers—the exam-correct instinct is "running systems get gradual daemon correction". VMs drift easily due to host scheduling, a context where chrony is preferred—worth remembering.
| Goal | Command | Key point |
|---|---|---|
| Show/set system time | date / timedatectl | timedatectl integrates it all |
| Write system→hardware | hwclock --systohc | --hctosys is the reverse |
| Change time zone | timedatectl set-timezone | Implemented via /etc/localtime |
| Check sync state | ntpq -p / chronyc sources | Shows peers and reachability |
Trap: "hwclock --systohc applies the hardware clock to the system" is wrong (that is --hctosys; parse systohc = system to hardware clock). And "fix drift on a live production server instantly with ntpdate" is wrong—sudden jumps (especially backwards) cause incidents; gradual daemon correction is the right answer.
4.1.3Section summary
- System clock (date/timedatectl) ⇔ hardware clock (hwclock; mind --systohc's direction); zones = zoneinfo → /etc/localtime link
- Sync via resident ntpd/chronyd (gradual); source = pool.ntp.org; verify with ntpq -p / chronyc sources
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Quick check
(just a quick review)Q1. You want to write the NTP-corrected system time into the hardware clock. Which command?
Q2. You must permanently set a systemd server's time zone to Asia/Tokyo. Which command?
Q3. A live production database server has drifted by a few minutes. What is the most appropriate fix?
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