What Is ISO 8601?
ISO 8601 is an international standard for representing date and time data, published by the International Organization for Standardization. It defines a clear, unambiguous way to write dates that eliminates confusion between formats like MM/DD/YYYY (US) and DD/MM/YYYY (UK). The standard uses a big-endian approach: year → month → day → hour → minute → second.
Date Formats
2024-03-01 # Calendar date (YYYY-MM-DD) — most common
2024-061 # Ordinal date (YYYY-DDD) — day 61 of 2024
2024-W09-5 # Week date (YYYY-Www-D) — Friday of week 9Time Formats
14:30:00 # Local time (HH:MM:SS)
14:30:00.456 # With fractional seconds
14:30:00Z # UTC (Z = Zulu = UTC)
14:30:00+09:00 # With UTC offset (e.g., Korea Standard Time)
14:30:00-05:00 # Negative offset (e.g., EST)Combined Date-Time (Most Common in APIs)
2024-03-01T14:30:00Z # UTC datetime (preferred)
2024-03-01T14:30:00+09:00 # KST datetime
2024-03-01T14:30:00.456Z # With milliseconds
2024-03-01T14:30:00.456789Z # With microsecondsDuration Format
ISO 8601 also defines duration notation with the prefix P:
P1Y2M3DT4H5M6S # 1 year, 2 months, 3 days, 4 hours, 5 minutes, 6 seconds
P30D # 30 days
PT1H30M # 1 hour 30 minutes (time-only duration starts with PT)Interval Format
2024-03-01T00:00:00Z/2024-03-31T23:59:59Z # Start/End
2024-03-01T00:00:00Z/P1M # Start/Duration
P1M/2024-03-31T23:59:59Z # Duration/EndWhy Use ISO 8601?
- Unambiguous: 03/04/05 could be March 4 2005, April 3 2005, or May 4 2003. ISO 8601 has only one interpretation.
- Sortable: ISO 8601 strings sort lexicographically in chronological order — critical for log files and databases.
- Machine-readable: Every major programming language can parse ISO 8601 natively.
- Internationally understood: Avoids the MM/DD vs DD/MM confusion between US and European conventions.
Parsing in Code
# Python
from datetime import datetime
dt = datetime.fromisoformat("2024-03-01T14:30:00+09:00")
# JavaScript
new Date("2024-03-01T14:30:00Z")
# Go
time.Parse(time.RFC3339, "2024-03-01T14:30:00Z")