Date and Calendar Information

A calendar is a system for organising the passage of time into named units such as days, weeks, months and years, allowing dates to be recorded and future events to be scheduled. Most calendars try to keep their reckoning in step with astronomical cycles, chiefly the day set by the Earth's rotation and the year set by its orbit around the Sun, but because these cycles do not divide evenly, adjustments are needed.

The seven-day week is an ancient convention; the English names of the days derive partly from Roman planetary deities and partly from Norse and Germanic equivalents, with the Sun and Moon naming two of them. The widely used Gregorian calendar, a refinement of the earlier Julian calendar, fixes the ordinary year at 365 days and inserts a leap day to compensate for the fact that the solar year is close to 365 and a quarter days. The leap-year rule adds a 29th of February in years divisible by four, except for century years not divisible by 400, which keeps the calendar closely matched to the seasons over long periods.

An astronomical calendar marks events tied to the sky, such as solstices, equinoxes, moon phases and eclipses, which shift slightly against the civil date from year to year. Leap seconds are a separate correction applied occasionally to coordinated universal time to keep clocks aligned with the Earth's gradually changing rotation, since that rotation is not perfectly constant. Together these mechanisms reconcile a fixed counting system with the irregular motions of the Earth.

Frequently asked questions

Why do we have leap years?

The Earth's orbit takes about 365.25 days, so adding an extra day roughly every four years keeps the calendar aligned with the seasons rather than drifting against them.

How does the Gregorian leap-year rule work?

Years divisible by four are leap years, except century years, which are leap years only if divisible by 400; this fine-tunes the average year length to match the solar year.

What is a leap second?

A leap second is an occasional one-second adjustment to coordinated universal time, added to keep precise atomic clocks in step with the Earth's slightly irregular rotation.