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Date windowing is the system by which full year numbers are converted to and from two-digit years. The year at which the century changes is called the pivot year of the date window. 
There are three major methods used to determine the date window:
- Fixed pivot year: There is a fixed pivot year
- Sliding pivot year: The pivot year is determined by subtracting some constant from the current year
- Closest date: Three different interpretations (last century, this century, and next century) are compared to the current date, and the closest date is chosen from the three.
Unix and windows systems have environment variables that set the fixed pivot year for the system. Any year after the pivot year will belong to this century (the 21st century), and any year before or equal to the pivot year will belong to last century (the 20th century). 
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