In an early scene, the high school science teacher produces a schematic diagram of the two nuclear bomb designs created by the Manhattan Project. He says, "Here are two designs based on plutonium." Actually, only the Fat Man bomb uses plutonium. The other, the Little Boy design, uses uranium.
After the six wires are cut to defuse the weapon, you can see the amount of time left on the clock is 07:16:45, which is the date of the first successful test of the Atomic Bomb.