Because firstly, the calendar is relatively late (and rather
difficult) invention. The West follows the Julean calendar, put in
place by Julius Ceasar in 46 BC. He established January 1st as the
start of the year and let the previous year drag on by 445
days, to correct the Roman calendar and synchronize it
with the position of the Sun. And that was only one of many
calendar systems in use. You can imagine how accurate it is to
place any event in those times.
Without a standarized calendar, it is very to say in which year an
event happened. For example, you may know your mother's birthday in
day, month and year. Now do you know her birthday in day, month and
year format in the Chinese lunar calendar. And don't just say it is
the year of the rooster, cos' that happens once every 12 years and
your grandchildren will never know which year of the rooster.
In ancient times, it is therefore common to date an event after the
reign of the king. That also happens in China...my grandfather was
born in the first year of the reign of the Emperor Xuantong...that
could be anytime between Dec 1908 to Nov 1909. We know Jesus was
born at the time Augustus Caesar ordered a census, when Quirinius
was ruler of Syria. You might say that the census year must
therefore be 1AD. But actually the census took a few years to
conduct and Quirinius might have been twice ruler of Syria. Our
historians are not particularly sure of their dates, and so it is
difficult to know when Jesus was born.
and our calendar takes the birth of Christ as the reference.
