🧭 Timezone

Timezones are defined and manged by IANA under BCP 175.

tip

Timezones change over time very often. The way you convert time between zone today, does not apply in the past or future.
Always use libraries.
Curious how one area's timezone change over time? Source code often denote history!

🇺🇸 United States

Common Name	IANA Identifier	SDT Offset	DST Offset
Eastern Time	`America/New_York`	−05:00	−04:00
Central Time	`America/Chicago`	−06:00	−05:00
Mountain Time	`America/Denver`	−07:00	−06:00
Arizona Time	`America/Phoenix`	−07:00	−07:00
Pacific Time	`America/Los_Angeles`	−08:00	−07:00
Hawaii Standard Time	`Pacific/Honolulu`	−10:00	−10:00

If your code touches timezones, these real-world cases are worth covering. Most naive implementations break on at least one.

Non-hour offsets — Asia/Kathmandu sits at UTC+05:45, Asia/Kolkata at UTC+05:30. Plenty of code assumes offsets are whole or half hours.
Non-hour DST jumps — Australia/Lord_Howe is the only timezone with a 30-minute DST transition (UTC+10:30 ↔ UTC+11:00); hourly cron jobs will not align with wall-clock hours across the transition. Antarctica/Troll uses a 2-hour DST jump.
DST at unusual times — America/Nuuk transitions at -1:00 (23:00 the previous day); older parsers may reject negative hours. Africa/Cairo and America/Santiago transition at 24:00 (next day); some parsers clamp to 23:59.
Lunar-calendar DST — Africa/Casablanca and Asia/Gaza pause DST during Ramadan, which follows the Islamic lunar calendar and drifts against the Gregorian year. Rules are hard-coded in tzdb through 2086.
Ambiguous abbreviations — CST maps to 66 different zones (China, Cuba, multiple Americas). Never identify a zone by abbreviation — use the IANA identifier.
Leap seconds — Most runtimes cannot represent a 61-second minute. Production systems typically smear leap seconds across a window rather than inserting them. Do not assume 23:59:60 parses.
UTC ≠ GMT — They are often conflated but are distinct. Use UTC in code and storage.