UTC (Coordinated Universal Time)
The world’s reference clock — the time standard every time zone is defined as an offset from.
UTC (Coordinated Universal Time) is the reference standard the world’s clocks are set against. Every time zone is defined as an offset from it, written like UTC+2 or UTC−5, so a clock anywhere on Earth can be translated back to a single shared point of reference.
Behind the letters
UTC is kept by atomic clocks, not by the sun or the seasons. Coordinating it is a joint effort across the standards world, with bodies such as the US National Institute of Standards and Technology (NIST) among those maintaining the atomic timekeeping that underpins it. That’s a deliberate design choice: atomic clocks tick with a precision no sundial or planetary observation can match, and a global standard needs a source that doesn’t drift depending on where you’re standing.
Not to be confused with
GMT (Greenwich Mean Time) is the older term, rooted in solar time measured at Greenwich. It survives today mainly as the name of the time zone used in Ireland and the UK during winter. For most everyday purposes, UTC and GMT read the same on a clock — the difference is what stands behind the number, atomic timekeeping versus the historical solar standard, not the number itself.
Local time is a separate idea again: it’s simply UTC plus your offset. Your watch shows local time; converting it to UTC (or to someone else’s local time) just means adding or subtracting the right number of hours. Offsets can also be large enough that two places sit on different calendar dates at the same instant — it can already be tomorrow in one time zone while it’s still today in another.