Time Zones

Core Idea

A timestamp is only useful when you know which clock rules produced it. Linux stores and displays time using the system clock, time zone settings, and locale formatting.

Commands To Try

date
date -u

date prints local time. date -u prints UTC. Course dates use the course-local timezone Asia/Singapore, even if a server, website, or external API displays another timezone.

Why It Matters

Logs, cron jobs, systemd timers, and git commits all depend on time. When debugging scheduled work, always ask: “Which timezone is this output using?”

Common Confusions

• UTC is not a country timezone; it is a reference clock.
• A date can change at different moments in different timezones.
• Cron and systemd timers use the machine’s local time unless configured otherwise.
• Web services may return HTTP Date headers in GMT.

Proof Check

Run date and date -u. If the hour differs, explain which one is local and which one is UTC.

Docs Pointers

• Run man date, then read format examples and -u.
• Run man timedatectl.
• Read IANA time zone database background if you want to know why timezone names look like Asia/Singapore.