The 24-hour clock

In your book, Maths Mela (Chapter 12), times are written in two ways. The 12-hour clock counts from 1 to 12 twice each day, so we add a.m. for times after midnight and before noon, and p.m. for times after noon. The 24-hour clock keeps counting after noon, from 12 up to 23, so it needs no a.m. or p.m. We write the word 'hours' after a 24-hour time.

The a.m. times, between midnight and noon, keep the same numbers. 01:30 a.m. is 01:30 hours, 05:30 a.m. is 05:30 hours, 06:30 a.m. is 06:30 hours, and 11:55 a.m. is 11:55 hours. Going the other way, 08:35 hours is 08:35 a.m.

For p.m. times from 1 o'clock to 11 o'clock, add 12 to the hour. 02:30 p.m. is 2 + 12 = 14, so it is 14:30 hours. 05:30 p.m. is 17:30 hours. 08:45 p.m. is 20:45 hours. 09:35 p.m. is 21:35 hours. 11:45 p.m. is 23:45 hours. The minutes never change.

To go back from a 24-hour time bigger than 12, take 12 away from the hour and write p.m. 19:30 hours is 19 − 12 = 7, so it is 07:30 p.m.