One year is about 365 days long (except in a leap year). It is the time it takes the Earth to go completely around (orbit) the sun once. A year is actually 365¼ days long, but a calendar has 365 days, except in a leap year.

The year starts on January 1 and ends on December 31 in the Gregorian calendar, but a fiscal year or a school year can start on a different day of the year.

There are several ways used to measure the length of a year.

• a solar year is based on the seasons. The Gregorian calendar is based on the solar year. The solar year is 365 days long.
• a lunar year is based on the moon and is usually 12 lunar months (29 days, 12 hours, 44 minutes each) or 354 days long.
• a tropical year is the time between two vernal equinoxes, the first day of spring when the lengths of daylight and night are the same.
• a sidereal year measures the time between when a selected fixed star is highest in the night sky.
• an anomalistic year is the difference between the times when the Earth gets closest to the sun.
• an eclipse year is the time between node passages. This is when the sun moves through a part of the sky where it is possible for the sun, Earth and moon to be in a line. It is also when solar eclipses can happen.

Solar and lunar years are used by different societies in daily life. The other measurements are used by astronomers.

• there was no year numbered "year zero" in a normal system of counting, because it would mean there is a year earlier than the first year, which was the year AD one in the Anno Domini system, also called 1 CE in the Common Era, used with our Gregorian calendar. However, some astronomers call the year 1 BC (or BCE) "year 0" to make it easier for them to count leap years before that year.