The problem is most likely due to having two different date formats in use in your data source (d/m/yyyy and m/d/yyyy - or a variation on those). US systems usually default to something based on m/d/yyyy but other regions typically use something based on d/m/yyyy. If you're using a UK system, for example, but are trying to input dates with a m/d/yyyy format, Excel won't know that a date input as 10/04/2012 is 4 October 2012 and not 10 April 2012 unless the cell is pre-formatted for d/m/yyyy dates. The converse also applies.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]