Date & Time Glossary

Short, plain-English definitions of the terms you meet across timekeeping, calendars and this site's tools.

AM / PM

From Latin ante meridiem (before midday) and post meridiem (after midday) — the two halves of the 12-hour clock.

Atomic clock

A clock that keeps time from the vibration frequency of atoms (usually caesium); the basis of UTC, accurate to nanoseconds per day.

Chronology

The science of arranging events in their order of occurrence in time.

Daylight Saving Time (DST)

The seasonal practice of setting clocks forward one hour in spring and back in autumn to extend evening daylight.

Epoch

A fixed reference moment from which time is counted — for example the Unix epoch, 1 January 1970 00:00:00 UTC.

Equinox

The two moments each year (March and September) when day and night are nearly equal everywhere on Earth.

GMT

Greenwich Mean Time — mean solar time at the Royal Observatory, Greenwich; historically the world reference, now effectively UTC+00:00.

Gregorian calendar

The internationally standard civil calendar, introduced by Pope Gregory XIII in 1582 to correct the Julian calendar’s drift.

Horology

The art and science of measuring time and making timekeeping instruments.

IANA Time Zone Database

The reference database (also called tz or zoneinfo) of the world’s time zones and their DST rules, used by most operating systems — and by this site.

ISO 8601

The international standard for writing dates and times: largest unit first, e.g. 2026-07-03T14:30:00Z.

Julian calendar

The calendar introduced by Julius Caesar in 45 BC with a leap year every four years; superseded by the Gregorian calendar.

Leap second

An occasional extra second added to UTC to keep it aligned with Earth’s slowing rotation; scheduled for phase-out by 2035.

Leap year

A year with 366 days: divisible by 4, except century years not divisible by 400.

Meridian

An imaginary line from pole to pole; solar time is defined by the sun crossing the local meridian at noon.

Military time

The 24-hour clock written without a separator (e.g. 1830 hours), used by the military and aviation.

NTP

Network Time Protocol — how computers synchronise their clocks over the internet to within milliseconds.

Perpetual calendar

A calendar (or algorithm) valid for any year, used to find the weekday of any date.

Prime meridian

The 0° longitude line at Greenwich, from which all time-zone offsets are measured.

Sidereal time

Time measured by Earth’s rotation relative to the stars; a sidereal day is about 23 h 56 m 4 s.

Solar noon

The moment the sun is highest in the sky at a location — rarely exactly 12:00 clock time.

Solstice

The two moments each year (June and December) when the sun reaches its greatest distance from the equator: the longest and shortest days.

Time zone

A region that shares the same standard time, defined as an offset from UTC.

Unix time

The count of seconds since 1 January 1970 00:00:00 UTC, used universally in computing.

UTC

Coordinated Universal Time — the world’s time standard, kept by atomic clocks and the successor to GMT.

Week number

The ISO 8601 numbering of weeks in a year (W01–W52/53), where week 1 contains the year’s first Thursday.

Zulu time

Aviation and military name for UTC+00:00, written with a Z suffix (e.g. 14:30Z).

Want the numbers behind the words? See Units of Time or the FAQ.