ISO Date Converter

Convert dates to and from ISO 8601 formats including week and ordinal notation

What is it and how does it work?

ISO 8601 is the international standard for representing dates and times. The key insight is that it uses a universally unambiguous format: YYYY-MM-DD for dates, T for the time separator, HH:MM:SS for time, and Z or ±HH:MM for timezone offset. "01/02/03" is ambiguous (January 2nd, February 1st, or February 3rd, 2001?), but "2003-02-01" always means February 1st, 2003. ISO 8601 is used in APIs, databases, log files, and any system that needs machine-readable, locale-independent timestamps.

This tool converts between ISO 8601 format and human-readable representations in any locale. Common conversions include: ISO → "February 1, 2003", adding timezone awareness (UTC, local, named TZ), extracting week numbers (ISO week date format: 2003-W05-6), and converting Unix timestamps (seconds since 1970-01-01T00:00:00Z) to ISO format. Understanding ISO 8601 is essential for working with APIs, scheduling systems, and international date handling.

Common use cases

• API response parsing: a REST API returns "2024-03-15T14:30:00Z" — convert to local time for display ("March 15, 2024 at 2:30 PM UTC" or "3:30 PM CET").
• Database timestamp: PostgreSQL stores timestamps as "2024-03-15 14:30:00+00" — convert to ISO 8601 full format with timezone for frontend display.
• Log file timestamp: a log file shows "2024-03-15T14:30:00.123Z" — the milliseconds (.123) and Z suffix indicate UTC, meaning this timestamp is unambiguous worldwide.

Frequently asked questions

What does the Z in ISO 8601 timestamps mean?

Z stands for "Zulu time", the military designation for UTC (Coordinated Universal Time). "2024-03-15T14:30:00Z" means 2:30 PM UTC. Alternatively, timezone offsets like +05:30 (India) or -08:00 (Pacific US) can be used. A datetime without any timezone indicator is "local time" — ambiguous and should be avoided in APIs.

What is the difference between ISO 8601 and RFC 3339?

RFC 3339 is a subset and clarification of ISO 8601, designed for Internet timestamps. It requires a timezone (Z or ±HH:MM), and allows only a hyphen (-) as a date separator (not dots or slashes). Most APIs use RFC 3339 when they say "ISO 8601". The formats are practically interchangeable in most contexts.

What is an ISO week date and when is it used?

ISO 8601 defines a "week date" format: 2024-W11-5 means 2024, week 11, Friday (day 5). ISO weeks start on Monday, and the first week of the year is the week containing the first Thursday of January. This format is common in business scheduling (fiscal week reporting) and is different from the US convention where weeks start Sunday.

How do I represent durations in ISO 8601?

ISO 8601 durations use the format P[n]Y[n]M[n]DT[n]H[n]M[n]S. Example: P1Y2M3DT4H5M6S = 1 year, 2 months, 3 days, 4 hours, 5 minutes, 6 seconds. PT30M = 30 minutes. P1D = 1 day. This format is used in XML, HTML (datetime attributes), and REST APIs for expressing time intervals.

Time

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