mtime and atime do not tell me how long the file has been on the server. ctime or the inode change time represents the time when the file's meta-information last changed. So ctime also doesn't tell me how long the file has been on the server.

Just thinking off the top of my head here: Could I come up with a script that logs any uploaded file in a database or textfile? The script would have to record the file the instant it gets uploaded. How could I do this if we are using ftp to do the uploading? (This idea isn't perfect, since the user could always rename the file, but whatelse could i do? . . .)