- The amount of time between the beginning of the first day of January and the end of the last day of December in the Gregorian calendar. (365 days, or 366 in a leap year.)
- The amount of time between corresponding dates in adjacent years in any calendar.
The average length of the calendar year in the Gregorian calendar is 365.2425 days (taking leap years into account). This is extremely close the average year length in other calendars such as the Islamic calendar based on the average time beween vernal equinoxes (currently 365.2424 days and increasing), but slightly different from formula-based calendars such as the Jewish calendar which averages 365.2468 days.