Copy files ignoring date stamp

I would like some help with the following issue:
I want to backup PST files, but only files that are really modified. Every time a users opens Outlook the PST file gets a new date stamp, although nothing changed. The file size is identical to the old one, but the 'new' PST file will be copied to the server, overwriting the old one cause of the new date stamp. Those PST files can be up to 3 or 4 gig, so that's a lot of traffic and the main thing: it takes a lot of time. 
 
I would like to write a script that rather looks at file size rather than date stamps.
- when the PST file has a different date stamp but same size: nothing happens.
- when the PST file has a different (smaller or bigger) size: overwite the server file.
 
Thanks!
 

So how do you currently look at the date? Please show us your code.

At the moment I run a batch script to copy the files with the xcopy function.

You might want to look at Robocopy. When a file is altered the OS sets the Archive Flag on that file. Robocopy has the ability to only copy files with the Archive flag set. And you can even tell it to reset the archive flag after the copy.

Thanks for the quick replies, I'll have a look at that.