Originally Posted by ND76

I've been noticing that Delta changes the time of departure of flights at its whim and caprice (for example, I was on a connecting flight at JFK with a scheduled departure time of 0800. When I landed at JFK, I looked at my phone to get the gate assignment for the connection, and it now had a departure time of 0750).

The GAs are under pressure from Delta management to get flights out on time. It is not surprising for them to make up their own rules, even though it would seem to be at odds with their announced practice (in your case, boarding until 15 minutes prior to scheduled departure).