Data Formatting Error
 

I am using Windows XP SP2 with MS Office 2007. I have a spreadsheet that is
in proper condition. After working on it, I save and close the spreadsheet
for later use. Upon opening the sheet at a later time, all of the cells that
had a data type of "number" are now converted to data type of "date". The
cells contain the correct values, however the cells display as "#####". I
can convert the cell type back to "number" and the values display correctly.
I cannot figure out why this is happening every time a save, close and then
reopen the file. Can any one help me with this?