"Man in the Desert" joke

Bill lived in the desert and everyday he had to walk three miles to get to the market.
One day as he was walking to the market he saw a man lying on the ground with a huge erection.
"What are doing?" asked Bill.
"I am telling time." said the guy.
"Oh really? What time is it?" replied Bill.
"It's 7:47." the guy responded.
Bill checked his watch and sure enough it was 7:47 "Wow, that's a great trick." He said as he continued to the market.
The next day on the way to the market Bill saw the same guy.
"Hi there, I see you are still telling time, so what time is it?"
The guy hesitated, then said, "It's 7:53."
Bill looked at his watch and sure enough it was 7:53. He congradulated the guy and once again headed to the market.
On the third day Bill saw the same guy, and again asked him the time and sure enough the guy got it right again. Bill went on his way to the market.
That night on his way home he saw the same guy and this time the guy was masturbating furiously.
"What the hell are you doing?" Bill asked.
The guy looked up at him, smiled and said, "I'm winding my watch!!"