iso-8601-vs-rfc-3339-reference

Overview

ISO 8601 and RFC 3339 are often used interchangeably but they are not the same. RFC 3339 is a stricter subset of ISO 8601 designed specifically for internet protocols. ISO 8601 has many optional variations (ordinal dates, week-dates, no separators, etc.) that RFC 3339 forbids.

Per RFC 3339 §5.6: it "defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard."

For APIs, always use RFC 3339. Parsers handle it predictably; ISO 8601 in full generality is parsing hell.

When to use