Understanding 12-Hour and 24-Hour Time

The 12-hour clock divides the day into two 12-hour periods: a.m. (ante meridiem): From midnight to just before noon. p.m. (post meridiem): From noon to just before midnight. Hours are numbered from 1 to 12. This means, 3:00 a.m. is 3 o'clock in the morning and 3:00 p.m. is 3 o'clock in the afternoon.

The 24-hour clock represents the entire day as one continuous cycle of 24 hours. Hours are numbered from 00 to 23. 00:00 (or 0000) is midnight. 12:00 (or 1200) is noon. For example, 09:00 is 9:00 a.m. (9 o'clock in the morning) and 21:00 is 9 p.m. (9 o'clock in the evening).

You’ll need to know how to convert between 12-Hour and 24-Hour time. Here’s how to do it:

• For a.m. times: add a leading zero if the hour is a single digit (e.g., 6:00 a.m. becomes 06:00).
• For p.m. times (except 12 p.m.): add 12 to the hour (e.g., 4:00 p.m. becomes 16:00).

You’ll also need to know how to convert from 24-Hour to 12-Hour time. Here are some quick tips:

• Hours 00 to 11: These are a.m. times. Remove the leading zero if there is one (e.g., 08:00 becomes 8:00 a.m.). However, it is important to note that 00:00 is 12:00 a.m. and 00:01 is 12:01 a.m.
• Hours 13 to 23: These are p.m. times. Subtract 12 from the hour (e.g., 17:00 becomes 5:00 p.m.). 12:00 to 12:59: This is 12 p.m.

Let’s do some questions to start with:

Convert 2:30 p.m. to 24-hour time.
Convert 10:15 a.m. to 24-hour time.
Convert 19:45 to 12-hour time.

Solutions: 14:30; 10:15; 7:45 p.m.

Calculating Time Durations: Adding and Subtracting

To add time, add the minutes. If the sum is 60 or more, carry over to the hours. Then, add the hours. For example, 2 hours 40 minutes + 1 hour 30 minutes = 40 minutes + 30 minutes = 70 minutes = 1 hour 10 minutes. 2 hours + 1 hour + 1 hour (carried over) = 4 hours. Result: 4 hours 10 minutes

To subtract time, subtract the minutes. If you can't subtract, borrow 1 hour (60 minutes) from the hours. Then, subtract the hours. For example: 5 hours 15 minutes - 2 hours 45 minutes. Borrow 1 hour (60 minutes) from 5 hours: 4 hours 75 minutes - 2 hours 45 minutes. 75 minutes - 45 minutes = 30 minutes. 4 hours - 2 hours = 2 hours. Result: 2 hours 30 minutes.

Finding Start or End Times

Here are quick formulas to help you find the end and start times:

• Start Time + Duration = End Time
• End Time - Duration = Start Time

For example, if a movie starts at 7:15 p.m. and lasts for 2 hours 20 minutes. What time does it end? Start Time: 7:15 p.m. Duration: 2 hours 20 minutes 7:15 p.m. + 2 hours = 9:15 p.m. 9:15 p.m. + 20 minutes = 9:35 p.m.

Let’s do some practice questions:

• Calculate the duration between 9:30 a.m. and 11:15 a.m.
• A train journey takes 3 hours and 40 minutes. If the train departs at 14:50, what time does it arrive?
• A concert ended at 10:25 p.m. If it lasted for 1 hour and 50 minutes, what time did it start?

Solutions: 1 hour 45 minutes, 18:30, 8:35 p.m.

Solving Time Problems in a Single Time Zone

In the exam, the questions that you might see related to time zone would likely be situation based and require the decision on how to formulate the calculation. This is the connection between the question provided and how to get to the answer. For worded problems, follow this process:

1. Read Carefully: Understand what the question is asking. Identify the given information (start times, end times, durations).
2. Choose the Right Operation: Decide whether you need to add or subtract time, or use a formula to find a missing value.
3. Convert if Necessary: Make sure all times are in the same format (12-hour or 24-hour) before performing calculations.
4. Check Your Answer: Does your answer make sense in the context of the problem?

Let’s do some harder practice questions now:

Question 1: John's Working Hours

The following table shows John's logged activities for a typical workday. He is paid based on his "Total Work Hours." This is when he does a “Work Session”.

John's employer only counts time spent in "Work Session" as paid hours, but wants to implement a new policy. The employer is considering including 50% of the "Travel to Work" time as paid hours. If this policy is implemented, what is the total paid work time would John receive each day? Round your answer up to the nearest minute.

There are two parts to answering this question:

Part 1: Total hours currently

1. Calculate the duration of Work Session 1: 12:00 - 08:30 = 3 hours 30 minutes
2. Calculate the duration of Work Session 2: 17:00 - 13:00 = 4 hours
3. Calculate Total Work Hours: 3 hours 30 minutes + 4 hours = 7 hours 30 minutes

Part 2: Additional Hours

1. Calculate the duration of "Travel to Work": 08:30 - 07:55 = 35 minutes
2. Calculate 50% of "Travel to Work" time: 35 minutes * 0.50 = 17.5 minutes. Round the additional paid time to the nearest whole number. This would be 18 minutes.

Then add both Part 1 and Part 2 together to get the total. 7 hours and 30 minutes + 18 minutes = 7 hours and 48 minutes.

Question 2: Movie Marathon Schedule

A cinema is planning a movie marathon with the following schedule. The schedule includes a break between each movie. For example, there is a break between Movie A and B, and between Movie B and C. Any additional movie would require a break before it.

The cinema wants to add another movie, Movie D, to the schedule. Movie D is 1 hour and 50 minutes long. They want to ensure the marathon ends at the same time as the original schedule (01:40 the next day) by adjusting the start time, and they will use a 5-minute break between each movie in the new schedule (between Movies A and B, B and C, and C and D).

What is the earliest time they must set for the start of the marathon to accommodate all four movies and still finish at 01:40 the next day?

Here’s how to solve this question:

1. Calculate the total duration of the current schedule: 01:40 - 19:40 = 6 hours.
2. Calculate the current total break time: 15 mins + 10 mins = 25 minutes
3. Calculate the new total break time: 3 * 5 minutes = 15 minutes.
4. Calculate the total duration of the new schedule: Original duration (6 hours) + Movie D duration (1 hour 50 minutes) - change in break times (25 minutes - 15 minutes = 10 minutes) =7 hours 40 minutes.
5. Calculate how much earlier the start time must be: 7 hours 40 minutes - 6 hours (original duration) = 1 hour 40 minutes. This means the new start time is 19:40 less 1 hour and 40 minutes = 18:00pm which is 6pm.

Practice time!

Now, it's your turn to practice.

The questions in this checkpoint are provided to give you an introduction to possible questions you may see in your exam. Don't worry too much as you'll continue to build your skills throughout the course.

Click on the button below and start your practice questions. We recommend doing untimed mode first, and then, when you're ready, do timed mode.

Every question has a suggested solutions videos after you complete the question. This video explains to you the steps to take to answer the question and provides tips and tricks.

Once you're done with the practice questions, move on to the next checkpoint.

Now, let’s get started on your practice questions.

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