Standards
The specs underneath it all — ISO 8601, RFC 3339, the IANA tz database, and friends.
ISO 8601
What the standard actually allows — and why that breadth bites interoperability.
RFC 3339
The strict, unambiguous internet profile of ISO 8601 — what your APIs should use.
Unix time / epoch
Seconds since 1970 UTC — simple, ubiquitous, and full of sharp edges.
IANA tz database
Where zone names and rules come from — and why you must keep it updated.