Mysterious times for departure/arrival

[FlyingConsultant]

Ok, this one has bugged me for a while. The example below is only one of many that I saw

I am picking up a friend of mine from UA750, Denver to EWR.

So I am checking on actual time from two sources. United.com and flightview.com. Here are the two results, pulled within seconds around 4:20pm EST.
United.com

Departure

--------------------------------------------------------------------------------

Airport: Denver, CO (DEN)
Scheduled departure: 12:15 PM
Actual: 12:36 PM
Reason: Schedule change due to Ramp Service
Terminal: Concourse B
Gate: B29

Arrival

--------------------------------------------------------------------------------

Airport: Newark, NJ (EWR)
Scheduled arrival: 5:58 PM
Estimated: 6:39 PM
Reason: Schedule change due to Ramp Service
Terminal: terminal a
Gate: 17
Baggage claim: --

Flightview.com

United Airlines
Flight Number 750 (UA750)
Departure
Airport: Denver, CO
Scheduled Time: 12:15 PM, Aug 10
Actual Time: 12:58 PM, Aug 10
Arrival
Airport: Newark, NJ
Scheduled Time: 5:58 PM, Aug 10
Actual Time: 6:27 PM, Aug 10 (Est)
Status: In air
Altitude: 37000 feet
Ground Speed: 457 KTS
Equipment: B752

I understand that reasonable estimates for arrival vary, although is is kind of funny. Does flighview make its own estimates? Where from? I always thought they pull actual data. But if that's true, where does UA get the estimate from?

And why do the departure times vary that much? who is reporting what here? Looks like UA reports push back and flightview actual TO - anybody knows?

When should I go to the airport now?

[Globaliser]

I'd bet that's exactly right. Out, off, on, in.

Same problem with ba.com and baa.com - one reports one set of numbers, the other one reports the other.

I would normally work on the assumption that the airline reports (and estimates) the "out" and "in" times, which are the official departure and arrival times, as the "in" time is the one that actually makes a difference for meeters and greeters - but then it's subject to more vagaries than the "on" time, as taxiing delays (or the lack of them) can add to the uncertainties over the touchdown time itself.

[Hartington]

I think you will find that Flightview pulls its' data from the FAA ATC system while United.com uses the United reservations system.

This means, that others have said, it's about the difference between out, off, on and in.