Another computer oddity today.
Had a call from a guy at work today wondering why every time he copied a set of data from one excel workbook to another the dates gained 4 years and a day.
Well it appears that all dates in windows are referenced by the number of days from 1st Jan 1900 (which makes sense). This is the default option for workbooks in Excel for windows. However, on macs, they're referenced from 2nd Jan 1904 (which makes less sense).
In this case the master file this guy was trying to copy from was based on a PC original file and hence was in the 1900 format and every new workbook he created on his mac defaulted to the 1904.
You can change the system a workbook uses by going Excel - Preferences - Calculation, then under workbook options select or deselect '1904 date system'. This only affects the current workbook.
The problem we had was that changing this to 1904 in the master workbook changed all the dates (that were right to start with) by 1 day and 4 years. To correct this you just need to subtract 1462 from each date. You can do this by writing this in an empty cell, then copying it, select all the dates to change, then edit - paste special, then paste = values and operation = subtract (if your dates are now too recent) or add (if they are too late)
You can find more detail
here.