FossilCoachman Men's Watch with Genuine Leather Bracelet Cuff

Q: How to change the date inside the watch ???

The crown has 3 positions, all the way in, half way out, all the way out. To adjust the date pull the the crown out to the second position. To adjust the time pull the crown out to the third position.

Q: Is this watch waterproof?

Went to Bermuda swam in the ocean and at the pool on the cruise ship. It absolutely is waterproof.

Q: What does the push button do? I noticed that the button at '10' starts/stops the second hand in "SEC" and the button at '20'' sends it back to 0.

Yes, your observation is correct.The button at '10' starts/stops the sec hand and the button at '20' sends it back to 0. The "Sec" is the seconds part of the stop watch. One rotation of sec hand moves the min hand in "Min". it is the stop watch feature of the watch. So the button at '10' starts your stop watch and the button at '20' resets the stop watchHope your qurstion is answered now.