Date modified of a file doesn't update to the most recent

Some files when I save them, the date modified in windows explorer doesn't
update. Also when I do a save, it asks me if I want to replace the existing
file with the latest file, but it gets confusing because the dates are the
same and won't update. Has anyone else had thses problems and if so has
Microsoft Vista come up with a fix. Thanks for your help in advace.

Tonyc