You shouldn't compare file creation dates for updates. The reason for this is that the file creation date will be completely different after it is decompressed (its file creation date would be the time and date that it was decompressed!)
Therefore you should check for updates using MD5 values. Check out this plugin
that creates file MD5's.
To do updates you will have to store the MD5 of your file in a file on the internet, and user's installers will download it and compare the MD5 in the downloaded file to their local file.
To make an MD5 for your update files, you should write a small NSIS installer to do it (using the plugin).