Time
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 (or 366 days in a leap 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
- A “:” is often displayed between the hours and minutes
- 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
Examiner Tip
- Even if allowed, put that calculator away for time-based questions!
- There is a button/mode on most makes/models of calculator that can help but it takes some getting used to
- It is usually as quick and as accurate to use non-calculator skills
Worked example