時こくと時間

An analogue clock has a short hour hand and a long minute hand. The short hand tells you the hour; the long hand tells you the minutes. The face is split into 12 marks, and each mark the long hand passes is 5 minutes. 'O'clock' is the long hand on 12, 'quarter past' is on 3 (15 minutes), 'half past' is on 6 (30 minutes), and 'quarter to' is on 9 (15 minutes before the next hour).

An ordinary clock splits the day into morning (am) and afternoon/evening (pm), counting 1 to 12. That is the 12-hour clock. The 24-hour clock counts straight through from 0 to 23. After 12 noon, you add 12 to the hour. So 3 pm is $3+12=15$, written 15:00, and 9 pm is $9+12=21$, written 21:00. Timetables for buses and trains use it a lot.

To find how much time has passed, it helps to count up to the next whole hour first. For example, from 9:15 to 10:00 is 45 minutes ($60-15=45$). Picturing the clock face and counting how far the long hand moves works well too.