I'm not quite sure what you're trying to do. When I modify a file's timestamp with touch(1), a "cvs update" does not list it as locally modified. Same if I make some changes to a file, save it, undo the changes, and save it again. So CVS does not seem to be using the timestamp. But I don't think it's doing a diff either. I suspect it is exchanging a checksum with the server or something like that.
So I'm saying that the behavior I think you want is what I'm seeing by default. I'm using a remote server (pserver) if that helps.
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