Unfortunately the SITE ZONE command that Peter mentioned won't help in this case as that command will only work for regular FTP connections.
The way a file's modification date/time is set after an SFTP file transfer is via the SSH_FXP_SETSTAT command passing in a new value for the mtime attribute on the file. Per IETF draft specification, this value must be a UTC time. Serv-U follows this spec and assumes this when parsing the value received from the client. Is it possible that your user's script is passing in a local time instead? That will affect the value that Serv-U eventually sets for the file.