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Dates may differ in two aspects: in the way that they are written (the format) and the calendar used.
In many parts of the world the Gregorian calendar is used, based on the solar cycle and consisting of 12 moinths and 365 days (366 days in leap years), and using a Christian event (the birth of Jesus) as a starting point.
Other calendars in use include the Hebrew calendar, the Islamic calendar, the Hindu calendar and the Chinese calendar. These are all based on lunar rather than solar cycles. They start at different points in history, have different month names and different month lengths. Their new years are at different times, and may shift each year in relation to the Gregorian calendar.
Date formats differ in two ways: the order in which the elements are written, and the punctuation marks used as separators. Some examples:
United States: mm/dd/yy or mm-dd-yy
United Kingdom, France: dd/mm/yy
Japan, Taiwan: yy/mm/dd
ISO 8601: yy-mm-dd
dd=date, mm=month, yy=year, expressed in numerals. yy may also be written yyyy to include the century.
Clearly, a date written 11/01/02, without any indication of the format, can indicate one of any three dates. Equally, input forms which request a date without specifying the format required, will collect data which is incorrect or can be incorrectly interpreted.
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