Time

Time Units

Understanding time requires knowing the standard units and how they relate:

Key fact: Time does not use a base-10 system — there are 60 seconds in a minute and 60 minutes in an hour, not 100. This is why you cannot simply move decimal points when converting.

12-Hour vs 24-Hour Clock

12-hour clock: uses am (midnight to noon) and pm (noon to midnight). The cycle repeats twice each day.

24-hour clock: runs from 00:00 (midnight) to 23:59 (one minute before midnight). No am/pm needed.

Converting Between Clock Formats

12-hour pm → 24-hour: add 12 to the hour (for times from 1:00 pm to 11:59 pm)

• 2:35 pm → 2 + 12 = 14 → 14:35
• 9:20 pm → 9 + 12 = 21 → 21:20

Exception: 12:00 pm stays as 12:00 in 24-hour; 12:00 am becomes 00:00.

24-hour → 12-hour: subtract 12 from hours greater than 12, add pm

• 14:35 → 14 - 12 = 2 → 2:35 pm
• 09:15 → hour is less than 12, so 9:15 am

Converting Mixed Time Units

Always convert to a single unit before calculating:

2 hours 45 minutes to minutes: 2 × 60 = 120 minutes + 45 = 165 minutes

3 days 5 hours to hours: 3 × 24 = 72 hours + 5 = 77 hours

195 minutes to hours and minutes: 195 ÷ 60 = 3 remainder 15 → 3 hours 15 minutes

Calculating Time Durations

The key technique is to work in steps through whole hours rather than trying to subtract directly (which leads to errors with the 60-minute system).

Method: counting on

Film starts at 19:50, lasts 2 hours 25 minutes:

• Step 1: add 2 hours → 19:50 + 2:00 = 21:50
• Step 2: add 25 minutes → 21:50 + 25 min = 22:15

Bus leaves at 08:47, arrives at 11:15 — find the duration:

• 08:47 → 09:00 = 13 minutes
• 09:00 → 11:00 = 2 hours
• 11:00 → 11:15 = 15 minutes
• Total = 2 hours + 13 + 15 minutes = 2 hours 28 minutes

Working Backwards

If you know the arrival time and duration, subtract to find the departure time.

Train arrives at 15:23, journey took 1 hour 48 minutes. When did it depart?

• 15:23 − 1 hour = 14:23
• 14:23 − 48 minutes: 14:23 − 23 min = 14:00, then − 25 min = 13:35

Departure: 13:35

Reading Analogue Clocks

• The short hand shows the hour
• The long hand shows the minutes (each small division = 1 minute; each large division = 5 minutes)
• The second hand (if present) is thinner and moves fastest

To read the time: read the hour hand first, then count the minute hand from 12 clockwise.

Timetables

Timetables use the 24-hour clock. To find journey time between two stops, subtract the departure time from the arrival time — working in steps if needed.

Example timetable:

Train A duration: 09:15 → 11:03

• 09:15 → 11:00 = 1 hour 45 min
• • 3 min = 1 hour 48 minutes

Common Mistakes

• Adding 12 to am times when converting to 24-hour (only add 12 to pm times, not am)
• Treating time like decimals: 2.5 hours ≠ 2 hours 50 minutes (it is 2 hours 30 minutes)
• Subtracting times directly: 11:15 − 08:47 ≠ 3:32 — always use the counting-on method
• Forgetting that midnight is 00:00, not 24:00

Tips and Tricks

• When calculating durations, count forward in steps: to the next whole hour, then the remaining hours, then remaining minutes
• For working backwards, reverse the steps: subtract hours first, then minutes
• Use a number line if you find time calculations confusing — mark the start and end, then hop through whole hours
• Double-check pm conversions: "is this afternoon/evening?" → add 12. "Is it morning?" → keep the same (except midnight = 00:00)
• 1 hour = 60 minutes, so 30 min = 0.5 hours, 15 min = 0.25 hours, 45 min = 0.75 hours