Last week, had a flight; IAD-DTW got a 30 minute delay since the flight arriving XXX-DTW that then was going to fly DTW-IAD was running 45 minutes behind. They flight left DTW for IAD 30 minutes late, but made up time en-route and only arrived a few minutes late. At T-30 from original departure (T-60) from the revised departure, then moved up the revised to now be T-40 (10 minute delay). In the end we departed at the original departure time.

I've learned in any delay situation, closely watch the inbound aircraft since sometimes they lag in making updates at both pushing out or pulling in departure times.