From Google:

On-time flight arrival is calculated by subtracting 15 minutes from the scheduled arrival time to determine the cutoff for being considered on-time. For example, if a flight is scheduled to arrive at 2:00 PM, any arrival before or at 1:45 PM is considered on-time, while an arrival at 1:46 PM or later is considered late. The official "arrival time" for this calculation is when the plane's doors are opened at the gate, not when it lands on the runway.