Concepts

Coordinated Universal Time (UTC) is the international time standard from which every time zone is defined as a fixed offset. UTC ticks at the steady rate of atomic clocks, with occasional one-second adjustments — leap seconds — that keep it close to the time told by the Earth's rotation.

Daylight saving time (DST) is the practice of advancing a jurisdiction's civil clock by one hour during the summer half of the year, then returning it to standard time for the winter half. DST changes only the numbers on local clocks; UTC, the Earth's rotation, and the length of the day are unaffected. The sun rises and sets at the same instants it always would, and the local clock simply reads those instants an hour later.

Unix time is a count of the seconds that have elapsed since 00:00:00 UTC on 1 January 1970 — a moment known as the Unix epoch. The same count is also called POSIX time, epoch time, or a Unix timestamp; the encoding is fixed by the POSIX standard and is the way nearly every computer represents an instant in time internally.

Solar noon at a place is the moment the Sun crosses the local meridian — the imaginary line in the sky running due north–south through the observer's zenith. The Sun reaches its highest altitude of the day at that instant and casts its shortest shadow; the same event is also called the solar transit or local apparent noon.

The equation of time is the gap between apparent solar time — what the Sun in the sky shows, on a sundial — and mean solar time, the steady time scale a clock keeps. It is positive when the Sun is ahead of the clock and negative when behind, and reaches up to about 16 minutes either way through the year, peaking near early November and bottoming out near mid-February.

A time zone is a region that observes a uniform civil time, defined as a fixed offset from Coordinated Universal Time (UTC). Standard offsets in use today range from UTC−12 in uninhabited US Pacific possessions to UTC+14 in the eastern Pacific, with several half-hour and quarter-hour offsets in between; the IANA Time Zone Database is the operational source of truth for which offset applies where, and how that has changed over time.

Twilight is the period before sunrise and after sunset when the Sun is below the horizon but the sky is still partially lit by sunlight scattered through the atmosphere. It is divided into three phases by how far the Sun has fallen below the horizon: civil twilight ends at 6° below, nautical at 12° below, and astronomical at 18° below — the threshold at which scattered sunlight drops below natural starlight and night begins for observational astronomy.