I don't know which criterion the airlines themselves use, but the MVT messages which are sent to the distribution systems show a 'landed' time and an 'arrived' time, the latter being the block time.
Example 1 : Last night's SA235 LHR - JNB where the MVT and the airport (ACSA) website show +/- the same landing time (1 minute difference)
* OPERATIONAL FLIGHT INFO * SA 235 -1 TU 02JUL
LHR ESTIMATED TIME OF ARRIVAL 0649 JNB
JNB AIRCRAFT LANDED 0643
ARRIVED 0652
SA235: London-Heathrow LANDED 6:44
Example 2: This morning's IB 3170 MAD-LHR where the airport arrivals and the MVT show the same 'landed' time of 0759.
08:20 IB3170 MADRID LANDED 07:59 Terminal five
* OPERATIONAL FLIGHT INFO * IB3170 0 WE 03JUL
MAD ESTIMATED TIME OF DEPARTURE 0645
LEFT THE GATE 0645
TOOK OFF 0655
ESTIMATED TIME OF ARRIVAL 0758 LHR
LHR AIRCRAFT LANDED 0759
ARRIVED 0815