ISO 8601 Date Format Explained

ISO 8601 — The International Date Standard

ISO 8601 is the international standard for representing dates, times, and time intervals. It was first published in 1988 and has become the de-facto format for data interchange across APIs, databases, and configuration files.

Core Format

The most common ISO 8601 representation is the extended format:

2026-02-28T14:30:00Z
2026-02-28T14:30:00+09:00
2026-02-28T14:30:00.123Z

Breaking this down:

• 2026 — Four-digit year
• 02 — Two-digit month (zero-padded)
• 28 — Two-digit day (zero-padded)
• T — Literal separator between date and time
• 14:30:00 — Hours, minutes, seconds in 24-hour format
• Z — UTC timezone designator (or an offset like +09:00)

Basic Format

ISO 8601 also defines a basic format without separators:

20260228T143000Z

This is less readable but more compact, used in some file naming conventions and compact data formats.

Timezone Designators

• Z — UTC (also called "Zulu time")
• +09:00 — Positive offset from UTC (e.g., JST)
• -05:00 — Negative offset from UTC (e.g., EST)

Why ISO 8601 Matters

1. Unambiguous — Unlike "02/03/2026" which could mean Feb 3 or March 2, ISO 8601 has one interpretation
2. Sortable — String sorting produces chronological order
3. Timezone-aware — The offset or Z suffix makes the absolute moment unambiguous
4. Machine-readable — Every major language has built-in ISO 8601 parsing

Language Patterns

Language	Pattern
JavaScript (date-fns)	yyyy-MM-dd'T'HH:mm:ssxxx
Python	%Y-%m-%dT%H:%M:%S%z
Java	yyyy-MM-dd'T'HH:mm:ssXXX
PHP	Y-m-d\TH:i:sP
Go	2006-01-02T15:04:05-07:00
C#	yyyy-MM-dd'T'HH:mm:sszzz