Visual RFC.
ALL RFCs / RFC 3339
Read the RFC ↗

RFC 3339 / START HERE

Internet timestamps, before the details

RFC 3339 is an Internet timestamp format, a restricted profile of ISO 8601. UTC means Coordinated Universal Time.

A timestamp identifies an instant using a calendar date, clock time and offset from UTC. It gives computers a consistent way to exchange times across locations.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Read the fields

  2. Locate the instant

  3. Compare carefully

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

Dates such as 10/11/1996 have different local meanings. This RFC specifies a predictable interchange spelling for protocol events; clock synchronization, recurring appointments and time-zone rule changes need other mechanisms.

A familiar example

A fictional log entry 2026-01-01T00:15:00+05:30 refers to 2025-12-31T18:45:00Z. The offset changes the displayed date, while the instant stays the same. Correct spelling alone does not prove the clock was accurate.

Helpful background

You can start here without reading another RFC. The guide introduces its essential concepts as you go.

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Read a complete timestampA timestamp identifies an instant. RFC 3339 defines a restricted ISO 8601 date/time format using the Gregorian calendar and an explicit relation to UTC, Coordinated Universal Time.
  2. Subtract the offset to find UTCThe signed offset is local clock time minus UTC. Finding the corresponding UTC date and time can cross a day or year boundary.
  3. Use the updated meaning of ZZero numeric offset does not by itself reveal the source’s local time-zone knowledge. RFC 9557 updates the original RFC 3339 interpretation of Z.
  4. Check calendar limits and leap-second contextCorrect punctuation is only the first check. Calendar days, clock ranges and leap-second events constrain which field values make sense.
  5. Sort strings only under compatible conventionsConsistently formatted timestamp strings can sort chronologically. Mixed offsets or fractional widths break the shortcut even when the timestamps are valid.
Start with chapter one →

This introduction explains RFC 3339 (2002). Read the original RFC ↗