Couple of points:
For Passenger Flights a Departure time shown in Reservation Systems and which in most cases is automatically passed to the DCS is normally the same as for the Operational Scheduled Departure time, but can be different (and on these very rare occasions is in my experience a +/- 5 to 15 minute range) and the reasons for it are tend to be 'cultural' and unless all involved in the Ground Handling are in the picture can lead to confusion.
All but one Airline and Systems Customer I have been involved in, the Operational Departure Time is when the Aircraft is off Blocks and Arrival is on Blocks. The exception used Take-off and Touchdown. But I cannot remember if this was reflected in the Scheduled Departure/Arrival times in the CRS/DCS.
One other difference that can be encountered is a true Shuttle operation where the Departure time is either the Operational Scheduled time, or when the Aircraft is full whichever is sooner.