Saved from a previous post:

One workbook was using a base year of 1900 and the other was using 1904.
(tools|options|calculation tab|1904 date system)

One way to add those four years back is to find an empty cell, put 1462 into
that cell.

Copy that cell.

Select your range that contains the dates. Edit|PasteSpecial|click Add (in the
operation box).

You may have to reformat the cell as a date (mine turned to a 5 digit number).
But it should work.

You may want to do it against a copy...just in case.

(I'm not sure which one you'll fix. You may want to edit|pastespecial|click
subtract.)

Most windows users use 1900 as the base date. Mac users (mostly??) use 1904 as
the base date.

DHT wrote:

I have created a document with several sheets, tracking video tapes and their
content, as well as date the tape was shot, etc. As projects are completed I
am saving the sheet as its own book and removing it from my original file.
I've done this to several sheets and each one, when I open the newly saved
file, screws up my dates and reverts to some date 4 years ago, and not even
the same day of the month. I ran into this problem a few years ago in a
similar circumstance. Can I save my document, or have I lost my original
date information? The orginal document is maintaining the correct dates.

--

Dave Peterson