Time Calculations

Reading timetables, calculating durations, and converting between units of time.

Units of Time

Relationship	Value
1 minute	60 seconds
1 hour	60 minutes
1 day	24 hours
1 week	7 days
1 year	365 days (366 in a leap year)
1 year	12 months

12-Hour vs 24-Hour Clock

12-hour	24-hour
12:00 am (midnight)	00:00
6:30 am	06:30
12:00 pm (noon)	12:00
3:45 pm	15:45
11:59 pm	23:59

To convert pm to 24-hour: add 12 to hours (except 12 pm stays as 12:00).

Calculating Duration

Count in steps: first to the next whole hour, then add the remaining minutes.

A film starts at 14:35 and ends at 16:10. How long is it?

$14{:}35 \to 15{:}00 = 25$ min

$15{:}00 \to 16{:}10 = 70$ min

Total: $25 + 70 = 95$ min $= 1$ h $35$ min

Converting Time Units

How many minutes are in 2 hours 45 minutes?

$2 \times 60 + 45 = 165$ minutes

Convert 200 minutes to hours and minutes

$200 \div 60 = 3$ remainder $20$

Answer: 3 hours 20 minutes

Real world: reading a train timetable

A student checks that the 08:47 train arrives at 10:23. Duration: $08{:}47 \to 09{:}00 = 13$ min, $09{:}00 \to 10{:}23 = 83$ min, total $= 96$ min $= 1$ h $36$ min. Timetable literacy is an everyday life skill built entirely on time calculations.

Real world: cooking a multi-course meal

A recipe requires a joint to roast for 2 h 45 min and rest for 20 min before serving at 18:30. Start time $= 18{:}30 - (2\text{h }45\text{min} + 20\text{min}) = 18{:}30 - 3\text{h }5\text{min} = 15{:}25$ . Meal preparation demands precise time arithmetic.

Real world: time zones and international calls

A London office (UTC+0) wants to call a New York client (UTC−5) during business hours (9 am–5 pm local). 9 am in New York is 2 pm London time, so the London team must schedule calls between 14:00 and 22:00 their time. Understanding time offsets is essential for global business.

Tip When subtracting times, count forward from the start time to the end time in two steps: first to a full hour, then to the final time. This avoids errors with borrowing minutes.

Practice Problems

A bus departs at 09:20 and arrives at 11:05. How long is the journey?

Show answer

09:20 → 10:00 = 40 min. 10:00 → 11:05 = 65 min. Total = 105 min = 1 h 45 min.

Convert 3:15 pm to 24-hour time.

Show answer

15:15.

School starts at 08:00 and a lesson is 45 minutes long with a 10-minute break. When does the second lesson end?

Show answer

First lesson ends 08:45. Break ends 08:55. Second lesson ends 09:40.