Many of us have spent our entire lives assuming that everyone in the world uses the same calendar -- not so. Turns out there are a LOT of calendars used worldwide. I probably learned about The Gregorian Calendar in first grade history class (!). The Jews use the Hebrew Calendar; Muslims use the Islamic (Hijri) Calendar; Iranians use the (modern) Persian calendar, and the Chinese use the Chinese Yin Calendar (not to be confused with the Chinese Yang Calendar).
our New Year's Day
|day||month||year||calendar adopted in|
|Gregorian||1||January||2008 C.E.||1752 CE|
|Islam||22||Dhu al-Hijjah||1428 A.H.|
For simplicity sake, I'm avoiding an explanation of the Chinese Lunar and Solar calendars, which are based on the ruling dynasty, among other things.
their New Year's Day (sorted chronologically)
|Christian Era 2008||1 January 2008|
|Islam 1429 (Muharram)||10 January 2008|
|Chinese 4705||7 February 2008|
|Persian 1386 (Norouz)||21 March 2008|
|Hindu 1929 (Ugadi)||6 April 2008|
|Hebrew 5769 (Rosh Hashanah)||29 September 2008|
I like using the ISO-8601 calendar for embedding into filenames, so that Untitled.txt becomes 2008-01-01-Untitled.txt -- that way, it's always obvious when the file was created, even if it's been emailed or copied. There's a nice Calendar Converter on Fourmilab which I used to generate this data.
Example: using this converter, I can see that Tiananmen Square protests of 1989 took place on Day 1, Fifth Month, Ji Si Year (Republic Dynasty). To me, it's still 4 June 1989.