Can I change the date and time APPX uses for testing date/time sensitive applications?

Yes, there is an environment variable called APPX FAKE TIME that you set to what looks like a starting date/time. Then run APPX and it uses this as the starting date/time for the session (i.e. the clock ticks forward from that date time as time rolls forward).

If you ...
         export APPX_FAKE_TIME="2001061510000000"
         appx


... you get a date/time of ...
         6/15/2001 8:00am


... and the clock ticks forward from there as time goes by.