The Star World Clock is a course in the Star World where Mario finds the Cosmic Crystals and Rockolanius (Starragus). The only entrance to the clock was to have both hands touch 12. If the hour hand touches either 3, 6, or 9, something else occurs. Otherwise, the person who tries to enter gets kicked out. After Black Bomber got Starragus (The half of Celestius that was imprisoned in there) out of the clock, the clock was free from the combination. Although this was also said to be true after Mario and his friends collected the Moon Crystals in The Cosmic Crystals III.
Note: Starman3 later made the bit about Rocko saying "WRONG!" when someone enters the clock with both hands on the 12, 3, 6 or 9 a joke within his videos, and applied it to the clock in the normal castle.