Number formatting lost
 

I have a workbook (EXCEL 2007) with about 30 sheets. It is tracking usage of
something, so many of the column are formatted to NUMBER, 0 decimal places.
I am careful to format the column cells and to save changes before I close
the work, but every time I open the workbook, I find that the numbers have
changed to JAN-00. It is easy to change them back again, but it takes time I
do not want to use in this way. I use lots of spreadsheets (all EXCEL) and
this is the only one that does this. Can someone please help?