The complete ISO 8601 guide.

Why ISO 8601?

Before ISO 8601, every region had its own date format. Is 01/02/2025 January 2 or February 1? Depends on whether you're American or European — and the format gives no clue. ISO 8601 fixes this with a globally unambiguous syntax based on three principles: largest unit first, zero-padded, and UTC by default.

The core format

2025-01-01T14:30:00Z
└────────┘ └──────┘└┘
   date     time   tz

The three parts are separated by T. The Z at the end means UTC (it stands for "Zulu time"). Without a timezone marker, the value is local time and shouldn't cross system boundaries.

Date formats

Form	Example	Meaning
Full calendar date	2025-01-01	January 1, 2025
Year and month only	2025-01	January 2025 (any day)
Year only	2025	The year 2025
Ordinal date	2025-001	1st day of 2025 (Jan 1)
Week date	2025-W01-3	3rd day (Wed) of week 1 of 2025

Time formats

Form	Example	Meaning
Hour, min, sec	14:30:00	2:30 PM exact
Hour, min	14:30	2:30 PM (no seconds)
With ms	14:30:00.123	2:30:00 and 123ms
With μs	14:30:00.123456	2:30:00.123456
Hour only	14	2 PM, no minutes

Timezone designators

Form	Example	Meaning
Z	2025-01-01T14:30:00Z	UTC
+HH:MM	2025-01-01T14:30:00+05:30	5h 30m east of UTC (India)
-HH:MM	2025-01-01T14:30:00-05:00	5h west of UTC (Eastern Time)
+HHMM	2025-01-01T14:30:00+0530	Same as +05:30
(none)	2025-01-01T14:30:00	Local time — ambiguous!

Durations

ISO 8601 also defines a syntax for durations using the P prefix:

P1Y          → 1 year
P1M          → 1 month
P1D          → 1 day
PT1H         → 1 hour
PT30M        → 30 minutes
PT15S        → 15 seconds
P1Y2M3DT4H5M6S → 1 year, 2 months, 3 days, 4h 5m 6s
P1W          → 1 week

The T separates date parts from time parts. M means "month" before T but "minute" after — this is the standard's main quirk.

Intervals

2025-01-01T00:00:00Z/2025-01-08T00:00:00Z   → start / end
2025-01-01T00:00:00Z/P1W                     → start / duration
P1W/2025-01-08T00:00:00Z                     → duration / end

Best practices

• Always include the timezone designator (Z or offset). Local times can't be safely shared between systems.
• Use UTC for storage. Convert to local time only when displaying to users.
• Prefer extended format (2025-01-01T14:30:00Z) over basic format (20250101T143000Z) — easier for humans to read.
• Sortable as strings. ISO 8601 timestamps sort correctly with plain text comparison: "2025-01-01" < "2025-12-31" works.
• Use it in JSON. The default for JSON.stringify(new Date()) in JavaScript is ISO 8601, which is exactly what you want.

RFC 3339 — the subset you actually use

If "ISO 8601" feels overwhelming, here's the good news: RFC 3339 is a strict subset that covers 99% of real-world use cases. It requires:

• Full extended-format date with hyphens: 2025-01-01
• T as the date-time separator
• Time with at least hours, minutes, seconds: 14:30:00
• Either Z or a ±HH:MM offset

That's it. If you produce RFC 3339, you're guaranteed valid ISO 8601 too, and it's universally parseable.

Try it

Paste any ISO 8601 string into the main converter to see how it parses, what Unix timestamp it represents, and how it looks in other formats.