Why are calendars different from culture from to culture?

Most cultures have their own historic calendar, as well as having the more accurate scientifically based calendar.
For instance the Chinese calendar started long before the standard calendar was established so the year the Chinese are on is not the same year as everyone else, of course they know the difference and celebrate the Chinese New Year as a tradition, not seriously thinking they are the only ones who have the right calendar.

It is good to remember your cultures traditions, along side the universal things of today.