The wall panel is related with all the other puzzles, so in the previous message before start the puzzle it tells you how to get all numbers but the last one and also the order to put them into the panel. There's a way to get that third number but first you have to get the other two. Or you can just try it randomly... but you'd know what happens if you fail.
That said, the key to solve clock's puzzle is what Angie says. All you have to do is to set the alarm clock's time correctly. You can take a look at the clock from the stats menu (that one with the character stats at left and another menu at right).
Just another thing in case you need it. There are three pieces of paper, but the last one is hidden ;)
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