Is ISO 8601 the same as RFC 3339?

RFC 3339 is a strict profile of ISO 8601 used by internet protocols. Every RFC 3339 string is valid ISO 8601, but ISO 8601 also permits forms (basic, ordinal, week dates) that RFC 3339 disallows.

What does the Z suffix mean?

Z is shorthand for the +00:00 UTC offset ("Zulu time"). 2024-03-15T14:30:00Z and 2024-03-15T14:30:00+00:00 are equivalent.

Can ISO 8601 represent BCE years?

Yes, via the expanded form with a sign and at least five digits, e.g. -0044-03-15 for 15 March 44 BCE. Note ISO 8601 uses astronomical year numbering, where 1 BCE is year 0.

Does the converter run locally?

Yes. All format generation happens in your browser, no server round-trip.

extended · basic · local · ordinal · year-week

Convert dates to ISO 8601 variants

Show every ISO 8601 representation of a date: extended UTC, basic, local-with-offset, date-only, year-week-day, and ordinal date.

Quick Start

How ISO 8601 works

ISO 8601 has multiple representations for the same instant — extended (with separators), basic (compact), with-offset (local + zone), and alternative formats like ordinal and year-week.

01
Paste any date
ISO, RFC 2822, or any format JavaScript Date parses.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
inputISO 8601RFC 3339epochlive
in · timestampany · ISO
```
2026-05-21T12:00Z
```
loaded · ISO 8601zone · ready
02
Six variants
Extended, basic, local-with-offset, date-only, ordinal, year-week.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via Intl.DateTimeFormat.
variantsUTC mathIntlDST-safelive
out · options6 · live
```
Z · basic · +05:30
```
options applied · UTCDST-safe
03
Year-week-day
ISO 8601's alternative: YYYY-Www-d. Useful for sprints.
Output respects the chosen timezone offset (Z vs +05:30) and pads digits per ISO conventions so sorts stay chronological.
weekoffsetzone-awaresortablepadded
week · computedW21 · ready
```
2026-W21-4
```
computed · instantsortable
04
Copy any one
Each row has its own copy button.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
copyclipboardno oracledeterministicclient-side
✓ · copiedready · done
```
✓ Copied
```
shipped · ready to shipsuccess

Pro tips

Time math without DST landmines

Always store UTC

Convert to local for display only. UTC storage avoids DST jumps and timezone drift in databases.

Watch leap seconds

POSIX time doesn’t count leap seconds; some systems do. Don’t use timestamps for sub-second precision across boundaries.

Prefer Temporal eventually

The Date API has many quirks. Temporal is shipping in modern engines and handles zones correctly.

Cron is timezone-tricky

Cron expressions run in the daemon’s local zone unless told otherwise. Pin a TZ explicitly for predictable runs.

ISO 8601 sorts naturally

YYYY-MM-DDTHH:mm:ss sorts correctly as a string. Don’t use US-style MM/DD/YYYY for anything machine-readable.

Beware Y2K38

Signed 32-bit Unix timestamps overflow in 2038. Use 64-bit storage end-to-end for long-lived systems.

Use Cases

When to use ISO 8601

APIs, logs, sortable filenames.

API timestamps

ISO is the JSON / REST standard. Math runs in UTC under the hood to dodge DST jumps — local time is only for display. That means scheduling logic and date diffs survive March / November fall-back without the classic off-by-one-hour bug.

DST-safe

Log files

Sortable, parseable, universal. Browser clock is the only source — no external time server, no privacy leak, no rate limit. Pair with NTP-synced server time for high-accuracy use cases, but for human-scale work the browser clock is plenty.

UTC-correct

Filenames

report-2026-05-21.pdf sorts chronologically. ISO 8601 output sorts naturally as strings, perfect for filenames (report-2026-01-15.pdf) and log lines that need chronological ordering without parsing. Drop into S3 keys for free time-bucketing.

ISO-sortable

Database

ISO strings are unambiguous in any column type. Pairs cleanly with cron, scheduled tasks, and timestamped log analysis. The tool's output drops directly into APScheduler, Linux cron, or GitHub Actions schedule expressions without re-formatting.

spec-grade

Cron / scheduling

Sprint codes via year-week-day. Output respects the chosen timezone offset (Z vs +05:30) so REST APIs and database TIMESTAMP WITH TIME ZONE columns round-trip cleanly. No ambiguity between "noon UTC" and "noon local".

zone-aware

International

Avoids 04/05 ambiguity (US vs EU). Confirms the difference between epoch seconds (10 digits) and milliseconds (13 digits) before you commit code that mis-multiplies by 1000 — a classic bug in cross-language time handling.

i18n

FAQ

Frequently asked questions

What's the difference vs RFC 3339?

RFC 3339 is a stricter profile of ISO 8601 used in internet protocols. Most APIs accept both.

Why offset over Z?

Local-with-offset preserves the speaker's clock context. Z is universal time only.

Year-week-day uses?

Sprint codes (2026-W21-4 = Thu of week 21), payroll periods, ordinal scheduling.

Ordinal date?

YYYY-DDD where DDD is the day of year (001-366). Common in aviation and astronomy.

Is anything sent to a server?

No. Parsing runs locally.

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