How to prevent Excel from modifying the file on exit?

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http://superuser.com – Whenever I open an Excel file and then close it without changing anything, the Excel would modify the file without asking a permission or giving any kind of warning. It modifies just 4 bytes at offset 0x046C, and I suspect this might be just the hard-coded last-modified time or something like that. The problem is that this really screws up the versioning control system, since the file's MD5 sum changes. Is there any way to stop Excel from doing this? As far as I can see there are no links of formulas or any other dynamic elements in the file. (HowTos)