Just to add to Chip's response...

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.

dwalesb wrote:

Why is Excel changing day & date data when I copy & paste? When I highlight
and copy a range of cells and paste into another sheet, in one column only
the dates of the material being copied (expressed as 1/23/4567) change. A
cursory examination seems to indicate a pattern of a one-day and four-year
change in each cell; e.g. 5/12/2005 becomes 5/13/2009. There have been some
variations of this where the day apparently stays the same but the year goes
back 4 years, e.g. 2006 becomes 2002.

During the same operation, although both sheets are in Arial font, the font
of all the material being copied is changed to Verdana upon being pasted.

--

Dave Peterson