Happy New Year's Day!
Of course, if you're still using the old Julian calendar, you'll have to wait until Jan. 14 to say that. But in another six days, on Jan. 7, you can finally celebrate the Eastern Orthodox Christmas. The old Julian calendar was replaced in 1582 by the modern Gregorian calendar now used almost everywhere around the globe.
The date Jan. 14 on our modern calendar is, in the Roman calendar, the first day of the year 2766 A.U.C. (ab urbe condita), counted from the founding of Rome. Although the Gregorian calendar, which says today is the first day of 2013, is used as the official calendar virtually everywhere, there are still many calendar systems being used today, primarily for religious purposes.
Like so many of the older calendars, the Chinese calendar is based on the positions of the sun and phases of the moon, what's known as a lunisolar calendar. Like all such calendars that define a month as one lunar cycle, there are extra days in the solar year, so they periodically add an intercalary, or extra month to keep the two in step. Our modern Gregorian calendar does that adding Leap Day every four years, but it's not because months are based on lunar cycles. It's due to the fact that the solar year is not equal to an exact number of days. Despite its name, the Chinese calendar is not used in China, but in Southeast Asia.
The Hebrew calendar is used in Israel and by millions of Jews around the world for religious purposes. The Islamic calendar helps Muslims around the world keep precise track of their religious celebrations.
The Persian calendar is used to this day in Iran (modern day Persia) and Afghanistan. It was officially adopted by the Iranian parliament on March 31, 1925. Unlike the three previous calendars, the Persian calendar is based on the solar year, like the Gregorian calendar of 365 days, with a 366-day Leap Year every four years, but the year begins on March 21 of the Gregorian calendar.
The Ethiopian calendar, used almost exclusively in that country, has 12 equal 30-day months with a 5-day intercalary period added at the end — six days in leap year.
The most oddball calendar still in regular use is the Balinese Pawukon calendar. It is 210 days long, so it is constantly out of sync with the Gregorian calendar. And to make matters worse, Balinese people use three calendars: the Pawukon, the Sashi, which is a 360-day lunar-cycle calendar, and the modern Gregorian calendar. The Balinese people must be great time jugglers!