testland/iso-8601-vs-rfc-3339-reference
Pure-reference catalog of the ISO 8601 vs RFC 3339 distinction. Covers the relationship (RFC 3339 is a strict subset of ISO 8601 designed for internet protocols), the syntactic differences (RFC 3339 disallows ISO 8601's '+02' offset short-form requires '+02:00'; RFC 3339 mandates a date-time separator T or space; ISO 8601 allows much more), the canonical date-time string format (YYYY-MM-DDTHH:MM:SS[.fff]±HH:MM or Z), per-language parser behaviour (Python isoformat, Java Instant.parse, JS Date.parse non-spec), and serialisation rules for APIs. Use when choosing a wire format, parsing third-party datetimes, or auditing time-string handling.
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parser-support.mdreferences/
Per-language parser support and anti-patterns
Parser-support matrix, version notes, and the anti-pattern table extracted from the core reference. The canonical format, forbidden forms, serialisation rules, pitfalls, and testable behaviours stay in SKILL.md.
Per-language parser support
| Language | RFC 3339 strict | ISO 8601 full | Tolerance |
|---|---|---|---|
Python datetime.fromisoformat | yes | partial | Accepts most RFC 3339 |
Python dateutil.parser | yes | mostly yes | Lenient |
Java Instant.parse | yes (RFC 3339 + Z) | no | Strict ISO 8601 subset |
Java OffsetDateTime.parse | yes | mostly yes | Lenient |
JavaScript Date.parse | platform-dependent | NO | Non-spec; varies by browser |
Rust chrono | yes | partial | Lenient |
Go time.Parse(time.RFC3339, ...) | yes | no | Strict |
.NET DateTimeOffset.Parse | yes | mostly yes | Lenient |
JavaScript Date.parse is the worst. Per
developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse:
"implementation-specific... The exact behavior of this function
varies between implementations." Use a library
(date-fns, dayjs) or the Temporal proposal where available.
Version notes
- Python
datetime.fromisoformat: full RFC 3339 (including a trailingZ) only from 3.11; earlier versions rejectZ. - Go:
time.RFC3339rejects fractional seconds; usetime.RFC3339Nanofor fractional. Temporalis a proposal; availability varies by runtime.
Anti-patterns
| Anti-pattern | Why it fails | Fix |
|---|---|---|
Date.parse('2026-05-20') in JS | Implementation-dependent (browser-local? UTC?) | Use a library; specify offset |
| Mix UTC and local-with-offset in same field | Sortability broken; consumer confusion | Pick one wire format |
Skip the offset (2026-05-20T14:30:00) | Ambiguous | Always offset |
| Storing local-format strings | Lose tz info; can't reconstruct | Store UTC + zone name separately if local matters |
4-digit milliseconds (2026...000) | Not all parsers accept | Use 3 (milli) or 6 (micro) digits |
| Sub-second precision but UTC string | Lose subsec on round-trip in some libraries | Test the round-trip |
Trusting Date.parse('5/20/2026') | US format; ambiguous | Use RFC 3339 always |
| Date-only without explicit time | Receiver-defined behaviour | Specify T00:00:00Z or document |