Wall clock Riddles

My Wall Clock
Riddle:
I have a clock in my house, on the wall. On a summer's day I forgot to wind it and it stopped. Then I went to visit a friend who had a watch that was always right on time. After I stayed for a bit, I went home, made a simple alteration and set the clock just right. Now how did I do this when I had no watch on me to tell how long it took me to come back from my friend's place?
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