Date and time formats (ISO 8601, time zones)
The YYYY-MM-DD format that sorts correctly, UTC vs time zones, and ISO 8601 durations like PT2H30M.
The ISO 8601 format
2026-09-11 date only: YYYY-MM-DD 2026-09-11T14:30:00 date and time, T separator 2026-09-11T14:30:00+02:00 with a time zone offset 2026-09-11T12:30:00Z Z = UTC (zero offset)
ISO 8601 sorts correctly in plain alphabetical order = chronological order (unlike 09/11/2026). That's why it's the format to use in file names, databases and APIs.
Date formats to avoid internationally
| Format | Ambiguous because... |
|---|---|
09/11/2026 |
day/month in most of the world, month/day in the US → 11 September or 9 November? |
11-09-26 |
2-digit year, order unclear |
Always prefer 2026-09-11, or spell the date out if the short form stays ambiguous: "11 September 2026".
UTC and time zones
- UTC (Coordinated Universal Time): the reference, with no daylight saving.
- An offset of
+02:00means it's 2 hours ahead of UTC. - France is at
UTC+1in winter (CET) andUTC+2in summer (CEST, daylight saving time).
ISO 8601 durations
P1Y2M10D 1 year, 2 months, 10 days PT2H30M 2 hours 30 minutes (the T separates the time part) P1DT12H 1 day and 12 hours
Week numbers (ISO 8601)
The ISO week starts on Monday, and week 1 is the week containing the year's first Thursday (so it sometimes starts in the previous December).
2026-W02 week 2 of 2026 2026-W02-3 Wednesday of week 2, 2026
The US "week number" convention counts differently (Sunday as the first day) — never mix the two conventions in the same system.
Thanks for the feedback!