Core Rules

GMT enforces a strict input/output contract to keep behavior predictable and auditable. These four rules are the non-negotiables.

Rule	Current behavior
String-first API	Public helpers consume ISO strings and return normalized strings where appropriate
Temporal-only internals	Temporal does the parsing and timezone math
Plain/zoned separation	plain/* is timezone-free, zoned/* is timezone-aware
No-throw public helpers	Invalid input returns a typed fallback instead of throwing

Sentinel returns

Public helpers never throw

Invalid input does not raise an error — it returns a typed fallback. If you are checking for thrown exceptions, you will never see one, and an unchecked sentinel will flow onward silently.

Invalid input fallbacks are consistent across the library:

Helper returns	Sentinel on invalid input
string	""
number	null
boolean	false
array	[]

Design philosophy

The rules above follow from four deliberate choices:

• Explicit inputs only. Public APIs accept clearly defined shapes — ISO 8601 date/time strings, IANA timezone identifiers, or Unix epochs as safe integers or digit strings, in seconds or milliseconds ({ epochUnit }). We do not attempt to parse arbitrary or ambiguous date formats.
• Predictable outputs. Helpers return normalized values (ISO strings, numbers, booleans, or arrays). Invalid input yields typed fallbacks ("", null, false or []) instead of throwing.
• No fuzzy parsing. Avoid “throw everything at the wall” patterns found in permissive libraries. If you need permissive parsing, perform it outside @northguild/gmt and then canonicalize to the strict shapes before calling into gmt.
• Developer comfort with standards. The library’s goal is to make developers comfortable and deliberate with ISO 8601, IANA timezones, UTC instants, and Unix epochs by keeping APIs small and explicit.