Time (Cambridge O Level Maths)

What do I need to know about time for IGCSE?

• Both 12-hour and 24-hour times could be used
• In the 12-hour clock system
  • AM is between midnight (12am) and midday (12pm)
  • PM is between midday (12pm) and midnight (12am)
• Times may have to be read from both analogue and digital clocks

• Times may have to be read from timetables

• Time does not work like the rest of the number system (based on 10s, 100s, etc) so calculations can get awkward
  • 60 seconds in a minute
  • 60 minutes in an hour 
  • 24 hours in a day
  • 7 days in a week
  • 365 days in a year
  • ... and many more !

• You should know the number of days in each calendar month, the following poem may help you remember...

How do I read a clock?

• A 12-hour clock goes round once for am and once for pm
  • am is midnight (12am) to midday (12pm)
  • pm is midday (12pm) to midnight (12am)
• A 24-hour clock uses four digits - two for the hour, two for the minutes
  • 1134 is 11.34am
  • The day starts at midnight which is 0000
  • 1pm is 1300, 2pm is 1400, ..., 10pm is 2200, 11pm is 2300

• Analogue clocks work in 12-hour time
• On the minute hand each number is worth five minutes
  • Some clocks will have markings for individual minutes
• The hour hand is always moving
  • At "half past" the hour hand should be halfway between two numbers (and the minute hand will be pointing at the number 6)

• Digital clocks can use either 24 hour time or 12-hour time
  • A “:” is often displayed between the hours and minutes
    • e.g.  1245 would be displayed as 12:45
  • am or pm does not need to be specified with 24-hour time
    • it may or may not be shown on a 12-hour time
  • For single-digit hours, clocks often miss out the first zero
    • e.g.  09:23 would be displayed as 9:23
• Timetables (for a bus or train for example) use the 24-hour time
  • Times are listed as four digits without the ":"

How do I calculate with time in terms of the 12-hour clock?

• Work in chunks of time
  • e.g.  calculate the minutes until the next hour, then whole hours, then minutes until a final time
• Ensure you know when the 12-hour clock switches from am to pm and vice versa
  • Remember midday is 12pm and midnight is 12am

How do I calculate with time in terms of the 24-hour clock?

• Work in chunks of time just like the 12 hour clock calculations
  • e.g.  Calculate the minutes until the next hour, then whole hours, then minutes until a final time
• If the hour is greater than 12, subtract 12 from it to find the 12-hour pm hour

How do I use bus and train timetables?

• These tend to use the 24-hour clock system
• Each column represents a different bus/train – these are often called “services”
  • e.g.  “The 0810 service from London King’s Cross”
• The time in each cell usually indicate departure times (when the bus/train leaves that stop/station)
  • The last location on the list usually shows the arrival time