What date is it?
You might think that the answer is obvious, but historically there have been many different calendars around the world and even today there are several different ones in use. The main differences between calandars are when one starts counting and whether one measures intervals by reference to the sun or the moon.
The most commonly used calandar dates time from the birth of Christ. Such dates used to be designated Anno Domini (AD) but are now often referred to as Common Era (CE). This calendar is based on a solar year of 365 days with a 366-day year every four years. It is used throughout the world including India and China.
However, in the Middle East, the Muslim calandar is widely used. This takes as its starting point the beginning of the Arab year in which the Hijra, the migation of Mohammed from Mecca to Medina, took place. In the Common Era calendar this is 16 July 622. Dtates in the Muslim calendar are designated as Anno Hegirae (AH). The Muslim calendar consists of 12 lunar months, 354 days in all, without adjustment to the solar year.
Then there is the Jewish calendar traditionally dated from the creation of the world. Dates using this system are designated Anno Mundi (AM). Like the Muslim calendar, it consists of 12 lunar months, but the Jewish one is adjusted to the solar year by the intercalation of an extra month at seven points in a 19-year cycle. It is 3761 years different from the Common Era calendar.
There is a Hindu calendar, a very old and very complicated system based on 12 lunar months with variations in different parts of India.
The Persian calendar is used in Iran and Afghanistan. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea.
You can find a detailed explanation of the various calendars here.