Reference
ISO 8601 and Unix Time
How ISO 8601 date-time strings and Unix timestamps represent the same instant, and where precision changes the meaning.
An instant can be written as an ISO 8601 string (2026-09-27T00:00:00Z) or as a Unix timestamp (a count of seconds since 1970-01-01T00:00:00Z). They describe the same moment; they differ in readability, precision, and range.
01Key facts
- ISO 8601 uses date and time fields with an optional UTC designator Z or a numeric offset.
- Unix time counts seconds since the Unix epoch and ignores leap seconds.
- Both represent instants, not calendar dates; a calendar date needs a time zone to become an instant.
| ISO 8601 (UTC) | Unix seconds | Unix milliseconds |
|---|---|---|
| 1970-01-01T00:00:00Z | 0 | 0 |
| 2026-09-27T00:00:00Z | 1790467200 | 1790467200000 |
02Limitations
Unix time is a number, not a UTC string, and does not encode a time zone or a leap-second count. ISO 8601 permits many valid forms, so parsers should be chosen deliberately.
Sources
- RFC 3339: Date and Time on the Internet: Timestamps — IETF (accessed 2026-09-27)
- ISO 8601-1:2019 — Date and time — Representations for information interchange — Part 1: Basic rules — ISO (accessed 2026-09-27)