# 1.11.1 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
• 24 hours in a day
• … and many more !

#### 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

• Digital clocks can use either 24 hour time or 12-hour time
• A “:” is often displayed between the hours and minutes
eg 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
eg 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
• eg 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
• 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
• eg. 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”
• eg “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

#### Exam Tip

Even when allowed, put that calculator away for time-based questions, they are pretty useless for these calculations!

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