Section 3 of RFC 3659 describes the
MDTM command for retrieving the file modification time. It doesn't say anything about which operations update the modification time, or when during those operations the update occurs. So it's up to the server OS. I would expect most operating systems to use the time of the last write to the file.
If you want the file creation time, you should use the
MLST command, and look for the
Create fact in the response. However, some operating systems may not be able to provide this. Most Unix filesystems don't store the file creation time, only the modification time, inode change time, and access time.