ISO 8601 Date Format — The Standard for Dates in APIs and Databases

ISO 8601 is the international standard (ISO 8601:2019) for representing dates and times. It eliminates ambiguity in data exchange — unlike “01/02/03” (is that January 2, 2003? February 1? Or February 3, 2001?).

Use the Timestamp Converter to convert between ISO 8601 and Unix timestamps.

Basic ISO 8601 formats

Date only:
  2026-05-12                    YYYY-MM-DD (recommended)
  20260512                      YYYYMMDD (compact, less common)

Date and time:
  2026-05-12T14:30:00           Local time (no timezone — ambiguous, avoid)
  2026-05-12T14:30:00Z          UTC (Z = Zulu = UTC+00:00)
  2026-05-12T14:30:00+05:30     UTC+5:30 (India Standard Time)
  2026-05-12T14:30:00-04:00     UTC-4:00 (Eastern Daylight Time)

With milliseconds:
  2026-05-12T14:30:00.000Z      Milliseconds (3 digits)
  2026-05-12T14:30:00.000000Z   Microseconds (6 digits)

Always include a timezone offset when communicating between systems. 2026-05-12T14:30:00 with no offset is ambiguous.

RFC 3339 vs ISO 8601

RFC 3339 is a profile of ISO 8601 commonly referenced in internet standards:

In practice: When APIs say “ISO 8601”, they usually mean RFC 3339 format: 2026-05-12T14:30:00Z. Use this format.

JavaScript: producing ISO 8601

const date = new Date('2026-05-12T14:30:00Z');

// Full ISO 8601 with milliseconds:
date.toISOString();           // "2026-05-12T14:30:00.000Z"

// Date only:
date.toISOString().split('T')[0];  // "2026-05-12"

// With timezone offset (not UTC):
// Use Intl.DateTimeFormat or date-fns-tz
import { formatISO } from 'date-fns';
formatISO(date);              // "2026-05-12T14:30:00Z"

Python: producing ISO 8601

from datetime import datetime, timezone

dt = datetime(2026, 5, 12, 14, 30, 0, tzinfo=timezone.utc)

# Standard ISO 8601:
dt.isoformat()             # "2026-05-12T14:30:00+00:00"

# With 'Z' suffix (more common in APIs):
dt.strftime('%Y-%m-%dT%H:%M:%SZ')  # "2026-05-12T14:30:00Z"

# Python 3.11+ accepts 'Z' in fromisoformat:
datetime.fromisoformat('2026-05-12T14:30:00Z')

# Earlier Python: replace 'Z' with '+00:00'
datetime.fromisoformat('2026-05-12T14:30:00Z'.replace('Z', '+00:00'))

ISO 8601 durations

Durations use the format P[n]Y[n]M[n]DT[n]H[n]M[n]S:

P = Period (required prefix)
Y = Years
M = Months (before T)
D = Days
T = Time separator (required when including time elements)
H = Hours
M = Minutes (after T)
S = Seconds

Examples:
P1Y        = 1 year
P3M        = 3 months
P7D        = 7 days
PT2H       = 2 hours
PT30M      = 30 minutes
P1Y2M3DT4H5M6S = 1 year, 2 months, 3 days, 4 hours, 5 minutes, 6 seconds

Used in APIs for cache TTLs, subscription intervals, and time-to-live values.

ISO 8601 intervals

Start/End:  2026-05-01T00:00:00Z/2026-05-31T23:59:59Z
Start/Duration:  2026-05-01T00:00:00Z/P1M  (1 month from start)
Duration/End:    P1M/2026-05-31T23:59:59Z  (1 month before end)

Used in calendar APIs, scheduling systems, and data range queries.

Why ISO 8601 for APIs

{
  "created_at": "2026-05-12T14:30:00Z",
  "updated_at": "2026-05-12T15:00:00.000Z",
  "expires_at": "2026-06-12T14:30:00Z",
  "duration": "PT1H30M"
}

Benefits:

• Sortable lexicographically — ISO dates sort correctly as strings
• Unambiguous — unlike MM/DD/YYYY vs DD/MM/YYYY
• Machine-readable — parseable by every language’s standard library
• Timezone explicit — no implicit assumptions about server timezone

Database storage

-- PostgreSQL:
-- Store as TIMESTAMPTZ (timestamp with timezone) — always UTC internally
CREATE TABLE events (
    id SERIAL PRIMARY KEY,
    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    event_date DATE NOT NULL  -- Date-only columns use DATE type
);

-- MySQL/MariaDB:
-- DATETIME stores without timezone; use TIMESTAMP for UTC auto-conversion
-- Or store as BIGINT Unix timestamp for portability

-- SQLite:
-- No native datetime type; store as ISO 8601 TEXT or Unix integer

Related tools

• Timestamp Converter — convert ISO 8601 to Unix and back
• JSON Formatter — validate API JSON with dates
• URL Encoder — encode ISO 8601 strings for query parameters