Acalendar year begins on theNew Year's Day of the givencalendar system and ends onthe day before the following New Year's Day, and thus consists of awhole number of days.

TheGregorian calendar year, which is in use ascivil calendar in most of the world, begins onJanuary 1 and ends onDecember 31.^[1] It has a length of 365 days in anordinary year but, in order to reconcile the calendar year with theastronomical cycle, it has 366 days in aleap year. With 97 leap years every 400 years, the Gregorian calendar year has an average length of 365.2425 days.

Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, theJulian calendar has an average length of 365.25 days, and theHebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar ("Islamic calendar") is alunar calendar consisting of 12lunar months in a year of 354 or 355 days.^[a] The astronomer'smean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the calendar year in most calendars.

Ayear can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year.^[2] This may be termed a "year's time", but is not a "calendar year".

The calendar year can be divided into four quarters,^[3] often abbreviated as Q1, Q2, Q3, and Q4. Since they are three months each, they are also calledtrimesters. In theGregorian calendar:

• First quarter, Q1: January 1 – March 31 (90 days or 91 days in leap years)^[4]
• Second quarter, Q2: April 1 – June 30 (91 days)
• Third quarter, Q3: July 1 – September 30 (92 days)
• Fourth quarter, Q4: October 1 – December 31 (92 days)

In some domains, weeks are preferred over months for scheduling and reporting, so they use quarters of exactly 13 weeks each, often followingISO week date conventions. One in five to six years has a 53rd week which is usually appended to the last quarter. It is then 98 days instead of 91 days long, which complicates comparisons.

In theChinese calendar, the quarters are traditionally associated with the 4seasons of the year:

• Spring: 1st to 3rd month
• Summer: 4th to 6th month
• Autumn: 7th to 9th month
• Winter: 10th to 12th month

The calendar year can also be divided into quadrimesters (from Frenchquadrimestre),^[5] lasting for four months each. They can also be called the early, middle, or late parts of the year. In theGregorian calendar:

• First quadrimester, early year: January 1 – April 30 (120 days or 121 days in leap years)
• Second quadrimester, mid-year: May 1 – August 31 (123 days)
• Third quadrimester, late year: September 1 – December 31 (122 days)

The calendar year can also be divided into semesters,^[6] lasting six months each and often being abbreviated as S1 and S2. In theGregorian calendar:

• First semester, S1: January 1 – June 30 (181 days or 182 days in leap years)
• Second semester, S2: July 1 – December 31 (184 days)