Flight Aware uses ATC time, not ACARS time. ATC time, as I understand it, starts and stops when somebody in the tower tells the computer you have lifted off or landed. If that specialist is busy cutting a new ATIS or downstairs using the facilities the times may be way off.
There isn’t a tower controller who stands there and “tells the computer” a departure time or arrival. Its pretty much automatic. When the radar acquires an aircraft’s beacon code and the track starts ( the data block acquires) the computer gets a departure message and the race is on.
You are right when radar can see you all the way to the ground, otherwise somebody has to tell the computer you are airborne, or landed as the case may be. I see this all the time at airports where low altitude radar coverage is non existent. In the rare case your transponder is inop or the wrong code is inserted a controller still has to tell the system you are airborne.
I also just remembered the FA times are dithered and may not be accurate to the minute. This is most likely the case in the OPs flights, ( BWI to ORD)
All I was trying to point out is the system is automated to a very great degree, and there isn’t a position in a tower assigned to press a button to tell a computer to send a departure message.
Controllers in a radar environment don’t have to send out a departure message. It happens automatically when the aircraft data block is acquired. And, yes, the rare wrong beacon code or the even more rare bad transponder would mean a controller would have to start a new track.
Oh, I didn’t take it as hostile. I guess it’s been a while since I visited a tower. I imagine things are a lot more automated now, I remember one of the controllers having to input some sort of arrival message to the computer when an IFR flight landed.