There is no
year zero in the widely used
Gregorian calendar, nor in its predecessor, the
Julian calendar. Under those systems, the year 1 BC is followed by AD 1. However, there is a year zero in
astronomical year numbering (where it coincides with the Julian year 1 BC) and in
ISO 86012004 (where it coincides with the Gregorian year 1 BC) as well as in all
Buddhist and
Hindu calendars.
There are two different ways of reckoning time
The first way of reckoning is the traditional one in historiography and in common usage to label years, centuries, and millennia via a counting method. The second is used, for example, with a person's age which reckons time according to a measuring system.
One way is to use cardinal numbers (e.g. one, two, three, ...) or ordinal numbers (e.g. first, second, third, ...) This corresponds to treating time as a discrete variable, and the labels as counts. Under this point of view, the first year counted after the starting point will come immediately after the first year counted before the starting point.
