GCSE Revision

Time calculations

12-hour and 24-hour clocks

The 12-hour clock uses am (midnight to midday) and pm (midday to midnight). The uses four digits and counts from 00:00 at midnight to 23:59.

  1. For times from 1 pm, add 12 to the hours: 4:35 pm is 16:35.
  2. For times between midnight and 1 am, the hours are 00: 12:20 am is 00:20.
  3. Morning times keep their hours, with a 0 in front if needed: 9:05 am is 09:05.

Example: write 4:35 pm and 7:10 am in 24-hour time, and 21:40 in 12-hour time.

  1. 4 + 12 = 16, so 4:35 pm is 16:35
  2. 7:10 am is 07:10
  3. 21 − 12 = 9, so 21:40 is 9:40 pm

Midday is 12:00, or 12 pm. Midnight is 00:00, or 12 am.

Key terms

Durations

A is how long something takes. Count on in steps, not by subtracting like decimals.

  1. Count on to the next whole hour.
  2. Count on whole hours.
  3. Count on the minutes that are left.
  4. Add the steps.

Example: how long is it from 10:45 to 14:20?

  1. 10:45 to 11:00 is 15 minutes
  2. 11:00 to 14:00 is 3 hours
  3. 14:00 to 14:20 is 20 minutes
  4. 15 + 20 = 35 minutes, so 3 hours 35 minutes

Harder example: a night shift runs from 22:50 to 06:15. How long is it?

  1. 22:50 to 23:00 is 10 minutes
  2. 23:00 to 00:00 (midnight) is 1 hour
  3. 00:00 to 06:00 is 6 hours
  4. 06:00 to 06:15 is 15 minutes
  5. 7 hours 25 minutes

Key terms

Hours, minutes and decimal hours

There are 60 minutes in an hour, not 100, so a time in isn't hours and minutes.

  1. Decimal hours to minutes: multiply the decimal part by 60.
  2. Minutes to decimal hours: divide the minutes by 60.

Example: write 2.4 hours in hours and minutes.

  1. The decimal part is 0.4
  2. 0.4 × 60 = 24 minutes
  3. 2 hours 24 minutes

Example: write 3 hours 36 minutes in hours.

  1. 36 ÷ 60 = 0.6
  2. 3.6 hours

Decimal hours are needed in speed calculations, such as dividing a distance by 3.6 hours.

Common mistake

1.5 hours is 1 hour 30 minutes, not 1 hour 50 minutes. Half an hour is 30 minutes.

Key terms

Timetables

In a timetable, each column is one journey and each row is a stop. Read down a column to follow one bus.

StopBus ABus BBus C
Hill Road07:4508:2009:05
Park Lane07:5808:3309:18
Station Square08:1608:5109:36

Example: Jo arrives at Park Lane at 08:25. How long does she wait, and when does she reach Station Square?

  1. Bus A left Park Lane at 07:58, so she needs Bus B, at 08:33.
  2. Wait: 08:25 to 08:33 is 8 minutes
  3. Bus B reaches Station Square at 08:51

Read the whole column: buses don't always take the same time between stops.

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