The times quoted in timetables, tickets etc. are "block" times. That is basically "chocks off" to "chocks on". Taxy times are incorporated (and adjusted for peak periods) as are prevailing winds for the route. Flying from airports such as JFK you can depart the gate on time, take-off an hour later and still arrive at your destination half an hour early! These days airlines allow plenty of elasticity in their schedules in order to mask delays.