To calibrate it you have to first set mph / km/h. to do this .. hold the left button (set) and the right (mode) down simultaneously for at least 6 seconds. the mph will appear, change it by pressing the set button. to move to the next step, press the mode button.
you will then be in the circumfrence mode. You must enter the circumfrence of your wheel that it will be attached to. Measure it in centimeters, and then enter it with the set button. when ready to move to the next part press the mode button.
now you can set the clock. set button to change the numbers and mode button to move to the next. After the seconds have been set press the mode button and the clock is set, your done.
use both buttons to reset the modes, but don't hold them more than 5 seconds or you have to go thru this again. good luck
av-100 owners manual