Signing MS-Excel spread sheets

[Berend Tober]

I hate to admit that I still use MS-Excel rather than an open source 
spread sheet tool, but workplace requirements constrain my fate...

Anyway, the problem with MS-Excel that I'm finding is that if I sign 
(detached signature) an MS-Excel XLS file, then me or someone else opens 
the file and then closes it even without making any changes, the 
signature no longer verifies successfully.  A binary comparison of the 
"after" file with a "before" copy confirms that the four or five bytes 
are changed by the simple fact of opening and closing the file. I figure 
it must be some kind of time stamp Excel records.

Has anyone else managed a work-around for this flaw? (Aside from the 
obvious -- "Stop using MS-Excel!" -- because that is a failure I cannot 
control...)