A clock is an element whose output cycles between 0 and 1 forever. You can set the cycle time (the time between consecutive 0 to 1, or 1 to 0 transitions), and also set the time the clock outputs a 1 (versus 0).
Set the cycle time by typing a number in the Cycle Time field or via the keypad.
Set the length of time the clock output should be 1 by typing a number in the One Time field or via the keypad. The one time must be less than the cycle time.
Click on OK to begin placing the clock, on Cancel to cancel it.