Updating a datapoint in a feed. What are the time parameters that a function in process_processlist.php would use?

Subject says it all.

I want to update some erroneous datapoints in a feed, on a fairly regular basis using a new function for the purpose in process_processlist.php. I can call the update_data() function in feed_model.php, 

         public function update_data($feedid,$updatetime,$feedtime,$value) 

but it is unclear, to me at least, what the exact significance is of the  $updatetime and $feedtime parameters. It sometimes looks like functions can use either or both almost interchangeably. If I identify the timestamp of an old datapoint in a feed, which of those parameters needs to have that timestamp, and what is the other one doing?