If other users can access the file, you cannot stop them from updating the timestamp on the file. If you open the file, then read or write the file you can get the file's mtime from a stat call, then when you are done with the file, you can reset filetimes with a utime call, using the data you got from stat.

All this sounds bogus to me. What are you trying to accomplish? not how to do what you are attempting to do that looks odd. It may be what you have to do, but I kind of doubt it.