Years before AD 1 are abbreviated BC for Before Christ or alternatively BCE for Before the Common Era.
Year numbers are based on inclusive counting, so that there is no "year zero".
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.
The word "year" is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc.
It was historically introduced in the 6th century and intended as counting years from the birth of Jesus.
The Anno Domini era is given the Latin abbreviation AD (for Anno Domini "in the year of the Lord"), or alternatively CE for "Common Era.
The ISO standard ISO 80000-3, Annex C, supports the symbol "a" (for Latin annus) to represent a year of either 365 or 366 days.