Check modified dates against a server's files

[LunaRebirth]

I have a program which sends it's file modified times in a directory to the server.
If the client modified times do not match the server's file times, the server sends the files to the client, ensuring that everyone gets the most up-to-date files (like images and models) and that they cannot edit them without uploading it to the server first.

This works fine, but if I package my program in a .zip file and send it to someone, the modified timestamps change to the current local time, so when they open the app, my server thinks all of the files are outdated.

What's the best approach to handle modified dates for this?

I tried making a .txt file that stores all dates of the files, and checking modification based on the text file, but it'd be super easy for someone to edit the .txt file to trick the server.

[MartinVee]

You should send the hash of the files instead of modification timestamps as these can be tampared as easily as a TXT file.

When you start your program, hash all the files (I'd suggest SHA or MD5 to avoid collisions), send all the hashes to the server, the server then compares the received hashes with the server's hashes, and send back only the files that don't match.

This has the added benefit of correcting an file corruption on the client's side. For extra performance, on the server side, re-hash only the files when they're updated. On the client side, though, re-hash each time.

[LunaRebirth]

Okay that's a much better solution than what I was doing.
Thanks!