It's just a clock going backwards with half the normal speed.

It generalizes to the formula r = -(n - 2)/2, so if we want a clock which is right every second, we could have a clock going backwards 43,199 the normal speed...

	▲	hleszek 3 days ago \| parent [-]
		Another possibility with this idea is to have a normal clock but with the 3 and the 9 numbers inversed (going backwards at normal speed). Such a clock will be right 4 times per day.