ISO 8601 or RFC 3339?
RFC 3339 is a strict profile of ISO 8601 for internet protocols. It requires a full date, a full time and an offset (Z or ±hh:mm). ISO 8601 allows more shapes: week dates such as 2026-W44-5, ordinal dates such as 2026-303, basic format without separators, and reduced precision. The outputs list includes UTC with Z, the selected zone's RFC 3339 form, and an RFC 9557 form that adds the IANA zone name in brackets so the zone survives a round trip.
Fractional precision and rounding
Choose 0, 3, 6 or 9 fractional digits, or Auto. Auto keeps the precision of the input and shows every non-zero digit. When fewer digits are requested, the rounding mode decides the result. Floor (the default) always moves toward the earlier instant, which matches how Unix tools truncate timestamps and keeps an event in the second it actually happened. Ceil, truncate toward zero and round-half-away-from-zero are also available. Rows that were rounded are marked so you never copy a rounded value by mistake.
Questions
- What does the Z at the end mean?
- Z (Zulu) is the designator for UTC, equivalent to +00:00. RFC 3339 also defines -00:00 to mean "UTC, local offset unknown".
- How many fractional digits should I use?
- Match the source precision: 3 for milliseconds, 6 for PostgreSQL/Python microseconds, 9 for nanoseconds. Extra trailing zeros are harmless, but dropping digits loses information.
- Is the offset the same all year?
- Not for zones with daylight saving time. The offset shown is the one in effect at the converted instant.