Hi, keep somewhere (say in a configuration file) the information about that last time you've run the job (or, more precisely, the last time stamp you've used). Anything newer than that time stamp should be considered, anything older can be discarded. Update the config file with the new time stamp when you are done.

Update: Loops was faster than me by a couple of minutes. Before answering the OP, I also considered the same solution as Loops's, but decided the one I suggested above was simpler. Now choosing between the two methods would probably require a bit more knowledge about the process.