Clock and Calendar Rules Flashcards

Formulas for clock angles and odd days in calendar questions.

Angle between clock hands

θ = |30H − 5.5M| degrees (H = hour, M = minutes).

How often do the hands coincide?

11 times in 12 hours; 22 times in a day.

Hands at right angles in a day

44 times.

Hands opposite (180°) in a day

22 times.

Odd days in an ordinary year

1 odd day.

Odd days in a leap year

2 odd days.

Odd days in 100 years

5 odd days.

Odd days in 400 years

0 odd days.

Leap year rule for century years

A century year is a leap year only if divisible by 400 (e.g. 2000 yes, 1900 no).