A calendar year is the 12-month period from 1 January to 31 December under the Gregorian calendar. It lasts 365 days, or 366 days in a leap year.
Length and structure
The calendar year contains 12 months of fixed length. February holds 28 days in common years and 29 days in leap years. The other 11 months hold either 30 or 31 days. A leap year occurs every 4 years, except for century years not divisible by 400.
Alignment with ISO 8601
ISO 8601 uses the Gregorian calendar as its underlying system. The calendar year and the ISO calendar year share the same start and end dates. They differ from the week-based year, which can shift by up to 3 days at the year boundary.
Calendar year vs financial year
The calendar year is distinct from a fiscal year. A fiscal year is any 12-month accounting period chosen by a government or business. The Australian financial year runs 1 July to 30 June. The US federal fiscal year runs 1 October to 30 September. Many corporations align their fiscal year with the calendar year for simplicity in reporting.