Calculate the number 2604

At 8 p.m. one night, a pilot who had run out of fuel made an emergency landing at a top-secret government base. He was quickly surrounded by security and taken inside to be interrogated. The interrogation was grueling because they wanted to make sure it was an unplanned landing and he was not a spy.
The interrogation lasted all night. At 6 a.m. they refueled his plane and let him go with his promise never to return. Four hours later he returned and landed again.
Security met him on the runway. They asked him why he had come back.
'I know I promised never to return but I brought my wife and now you have to tell her where I was all night...'