ZipKit
Glossary

UTC

UTC (Coordinated Universal Time) is the primary time standard the world uses to regulate clocks and time zones, maintained using a network of atomic clocks and occasional leap-second adjustments to stay aligned with Earth's rotation. Every time zone is defined as a fixed or seasonal offset from UTC (e.g. UTC+2, UTC−5), and it carries the reference offset +00:00 in date/time formats.

How it works

UTC deliberately does not observe daylight saving time — it's a single, constant reference that other zones offset from, which is exactly why servers, logs, and APIs standardize on it internally: a UTC timestamp never has the twice-a-year ambiguity a local time zone with DST has (the hour that repeats or disappears at the clock change). "GMT" (Greenwich Mean Time) is often used interchangeably with UTC in casual writing; they agree to within a second in practice, but UTC is the formally defined standard, while GMT is technically a specific time zone (UTC+0 in the UK during winter).

Example:

2026-09-29T14:30:00Z        (Z suffix = UTC, per ISO 8601)
2026-09-29T10:30:00-04:00   (same instant, US Eastern Daylight Time)

Common pitfalls

  • Storing timestamps in local time instead of UTC makes any later time-zone conversion, DST handling, or cross-region comparison unreliable — the near-universal best practice is to store in UTC, convert to local only for display.
  • A date-only value like 2026-09-29 with no time or offset is ambiguous about which time zone's "today" it means — a common source of off-by-one-day bugs near midnight.
  • UTC and GMT are close enough to be used interchangeably in everyday conversation, but code that needs to be precise (or handles historical dates, or leap seconds) should treat them as formally distinct.
  • Leap seconds mean UTC is not perfectly uniform — extremely time-sensitive systems (network time protocol implementations, some financial systems) have to handle the rare inserted leap second explicitly.

Related terms

  • ISO 8601 — the string format that represents a UTC instant with a trailing Z.
  • Unix timestamp — a UTC-based, timezone-free count of seconds since the epoch.

See also