Practical International Data Management - Dates

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

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
      ANSI: yy.mm.dd
      United Kingdom, France: dd/mm/yy
      Germany: dd.mm.yy
      Italy: 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.

Considerations

• The International Data Line moves.
• Example: Samoa shifted sides in 1892, then again in 2011.
• Twins don't necessarily have the same birthday.
• Example: Twins born 24 days apart (in Dutch): Tweeling scheelt 24 dagen (Cached version)
• Example: Twins born in different years: Twins born in different years (Cached version)

Processing

• Give great thought to the way forms collect date information to prevent wrong data being collected or poorly interpreted.

• Do not assume that the whole world uses the same format or that your customer will know what format you require.

• Weekends, holidays and special days, such as Christmas, Mother's Day and so on, are not universally celebrated, and where they are they are celebrated on different days.
• Resource - Humbug?: Graham Rhind discusses how holidays fall on different dates in different places, and how we make assumptions about who is celebrating them, and when, at our peril. January 2013. (Cached version)

• Weekends and holidays are not fixed.
• Example: Oman and changed their weekends from Thursday and Friday to Friday and Saturday in June/July 2013 to align themselves to other countries in the region.

Resources

• Humbug?: Graham Rhind discusses how holidays fall on different dates in different places, and how we make assumptions about who is celebrating them, and when, at our peril. January 2013. (Cached version)

Practical International Data Management Online.  A free resource from GRC Data Intelligence. For comments, questions or feedback: pidm@grcdi.nl