I had a flight out of TUL once (DL partner regional carrier... I forget which one) that was scheduled to leave about 5:00 PM.
At about 10:00 AM, FlightStats posted that the flight was going to be an hour late.
When I got to the airport, local staff agreed that the flight was going to be an hour late.
Even after departure time had long passed, the DL system did not show the flight delayed.