ISO 8601 Date Converter

ISO 8601 Date Converter

Updated August 20, 2026

ISO 8601 converter online. Parse ISO dates and durations like P3DT4H. Free ISO 8601 date format tool.

As duration

Not a valid ISO 8601 duration (example: P3DT4H5M).

As instant

Enter a timestamp, ISO date, or type "now".

Features

  • Parse ISO 8601 datetimes and durations (P3DT4H)
  • Show the instant in UTC and Unix
  • Explain duration as days/hours/minutes/seconds
  • Accept common RFC 3339 timestamps
  • Flag strings the JS parser will not accept

Common Use Cases

  • Decode a Duration from an API (P1DT2H)
  • Check whether a timestamp needs a Z or offset
  • Convert ISO week-unrelated instants to Unix
  • Teach the difference between duration and interval
  • Validate a JSON date field before it hits a client

ISO 8601 is a family, not one string

Calendar dates, times, offsets, and durations are different productions. 2026-08-20T12:00:00Z is an instant. P3D is a duration with no start. JavaScript parses many instants and almost no durations. This page handles both.

Examples

Valid - Instant
2026-08-20T21:00:00.000Z
Valid - Duration
P3DT4H5M
Invalid - Invalid duration
3 days

Frequently Asked Questions

Are months in durations exact?

We approximate P1M as 30 days and P1Y as 365 days. Civil months vary. For billing, use calendar arithmetic on the calculator page.

Why did my local ISO without Z parse as local?

ES5 treats date-only as UTC and date-time without offset as local, inconsistently across engines. Put a Z or offset on purpose.

Related Tools

Date calculator

Add calendar units

Try it

Duration

Difference between two instants

Try it

Unix timestamp

Epoch forms

Try it

Tips

  • Always include a timezone or Z on instants you send over the wire.
  • Duration is not the same as two ISO dates with a slash (intervals).