Modification time is available in a variety of ways, and which one you use depends on how you work. The OS shell itself can give you file modification time in its directory listings, ftp clients can list the same information, and through PHP there is the [man]filemtime[/man] function (which uses the information supplied to it by the OS). And as Frederick pointed out, a lot of site editors also track modification times. Whichever way you work: the files to be uploaded are those with modification times later than the time you last uploaded 🙂