The client connecting to you can control this. Usually, there is an option for them to "preserve Timestamp" when they "PUT" the file onto your system. If you can get your 3rd party to do this, then the option you are using below will suffice and actually have the timestamp of the file from the originating system.
Otherwise, you'll have to connect back to them and get a file listing of the file transferred then retrieve the last modified date from that connection on THEIR system.
Hope this helps...