ISO 8601 specifies dates like `2026-05-20`, times like `15:30:00`, and combinations like `2026-05-20T15:30:00Z` (the Z means UTC).
The format sorts correctly as text, no parsing needed to order events chronologically. It's also unambiguous: `05/06/2026` could be May or June depending on country; `2026-05-06` always means May.
Use it everywhere. JSON APIs, log files, URLs. Most languages have built-in parsers.