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Excel recognises two date systems; 1900 and 1904 and calculates dates as days elapsed since 1st January 1900 or 1st January 1904. The default setting on Windows Excel is 1900 and the default setting on Macintosh Excel is 1904, although both operating systems support both date formats. The spreadsheets linked from this page were created on an Apple Macintosh computer set to the 1900 system.
Neither system supports ninteenth century and earlier dates and that is why there are three date columns in the spreadsheets. The YYYYMMDD column is for sorting into chronological order, the day/month column and the year column together are for easy reading of a date. Only the day/month column is formatted as dates, all the other cells are general text format.
If you find that the day/month data varies by one day from the YYYYMMDD data your computer is set to the 1904 date system. (The variation is actually four years and one day but the year is hidden in the chosen display style of 14-Mar and is almost certainly wrong anyway!)
To change from 1904 Date System to 1900 Date System - (the change should happen immediately you click Save or OK):
AppleMac
With Excel open click on Excel in the top menu bar -> Preferences -> Calculation -> untick 1904 Date System -> OK
Windows
With Excel open click on tools in top menu bar -> Options -> Calculation -> untick 1904 Date System -> Save.
For more information about the two date systems visit Microsoft Excel support.