Create An Audit Of A Folder Structure, And Tell Is/what Has Changed?

I've got an application that takes the files in a folder structure and publishes them to another location. Currently it goes through every folder/file and compares the dates in the source and destination. If the file doesn't exist in the destination, or is older, then it copies it across from the source. It's obviously foil proof, but not as fast as it could be.

Ideally, I'd like to create an audit (or audits) saying what the current state of play is in the destination and compare the source folders/files against that instead.

This of course assumes the destination isn't messed around with manually, but I think that's a fair limitation.

So has anyone got any suggestions for how this audit could work? ie: How I could store data/values, such that on the next run when I look down the source folder/files I could compare it to the audit and see 99% of the files are unchanged, so only send across the X new ones etc?

ps: I don't suppose there's a valid (in Windows) that can tell you the date any file in a folder was created/amended? ie: So rather than checking down 200 files to see their individual dates/times, if I could get a single folder level value saying the date of last update/create, if it hasn't changed since the last run, there's no work to do! There is GetLastWriteTime but this doesn't seem to get updated if you amend/update a file, rather than creating a new one!