When computations are done involving both years AD and years BC, it is common to use Astronomical year numbering, in which 1 BC is designated 0, 2 BC is designated −1, and so on.
Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter.
In tropical and subtropical regions several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.
The Persian calendar, in use in Afghanistan and Iran, has its year begin at the midnight closest to the instant of the northward equinox as determined by astronomical computation (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.