The "last report" date should be the time the most recent report arrived at the server, regardless of any client-side embedded dates. This allows event correlation even in the face of client clock drift. (In a perfect world, the clocks would all stay in lockstep. Unfortunately, this isn't a perfect world and ntpd will bail when the drift is too high - exactly when it is needed most.)

For example, when 10 hosts fail you can go back and say "oh. they all failed at 2:10am. That is when the fileserver backup runs. It must be causing puppet timeouts" rather than "hmm. so they failed between 2 and 3, but only 1 had trouble during the backup."

Perhaps "Report Date" is a better name for the embedded datestamp, since that is the date the report was generated (so importing older reports will not have unexpected side-effects.)