Overview
UTC is the modern time standard used by software systems. GMT is a historical timezone and civil-time label. In many everyday contexts they show the same clock time, but developers should prefer UTC when describing stored instants, API payloads, logs, and Unix timestamps.
This guide focuses on practical implementation choices: how values should be stored, how they should cross API boundaries, and where developers most often introduce timezone or precision bugs. The safest pattern is to keep the stored value unambiguous, document the unit, and format the date only when it reaches a human-facing interface.
When to use it
Use UTC for storage, server logs, token expiry, scheduled instants, and timestamp conversion. Use regional IANA timezone names when a business rule depends on local civil time.
For production systems, also consider how the value will be indexed, logged, serialized, and read by other teams. A timestamp field that is obvious in one programming language can become ambiguous when it is consumed by JavaScript, SQL, mobile clients, or third-party integrations.
Developer examples
Use examples like this as a starting point, then adapt the timezone and precision to your application contract.
const now = new Date();
console.log(now.toISOString()); // UTC instant
console.log(now.toUTCString());Common pitfalls
Do not use GMT as a substitute for every timezone problem. If you mean London local time, use Europe/London because daylight saving rules can differ from a fixed UTC-style instant.
Most timestamp bugs come from hidden assumptions: local time treated as UTC, seconds treated as milliseconds, formatted strings parsed without offsets, or narrow integer columns copied from old examples. Add tests for boundary dates and document the expected unit beside every external timestamp field.
FAQ
Are UTC and GMT the same?
They often display the same clock time, but UTC is the standard developers should use for precise timestamp work.
Do Unix timestamps use UTC?
Yes. Unix timestamps count elapsed time from 1970-01-01 00:00:00 UTC.