Right button toggles through the hours and minutes. Left button toggles through month, day and seconds, center button is used to add units. So, press/hold the right button, and the hours will blink, then you can use the center button to increase hours. Press the right button again to go to minutes, then use center button to increase minutes. Left button, date will blink, increase months with center button, press left button again, increase days with center button, press button again, and you can reset seconds to 0. Press left button once more, and you are all set. Press/hold center button to switch from C to F. I don't think you can change from 24h to 12h, at least I haven't found a way.