Adding and Subtracting Time
When does the movie end? How long is that flight? When to start cooking dinner? We often need to add or subtract time!
Add or subtract the hours and minutes separately.
But we may need to adjust for minutes above 60 or below 0.
Adding Times
Follow these steps:
- Add the hours
- Add the minutes
- If the minutes are 60 or more, subtract 60 from the minutes and add 1 to hours
Like this:
Easy example: What is 2:45 + 1:10 ?
Add the Hours: 2+1 = 3
Add the Minutes: 45+10 = 55
The minutes are OK, so the answer is 3:55
Hard example: What is 2:45 + 1:20 ?
Add the Hours: 2+1 = 3
Add the Minutes: 45+20 = 65
The minutes are 60 or more, so
- subtract 60 from minutes (65−60 = 5 Minutes)
- and add 1 to Hours (3+1 = 4 Hours)
The answer is 4:05
Subtracting Times
Follow these steps:
- Subtract the hours
- Subtract the minutes
- If the minutes are negative, add 60 to the minutes and subtract 1 from hours
Like this:
Easy example: What is 4:10 − 1:05 ?
Subtract the Hours: 4−1 = 3
Subtract the Minutes: 10−5 = 5
The minutes are OK, so the answer is 3:05
Hard example: What is 4:10 − 1:35 ?
Subtract the Hours: 4−1 = 3
Subtract the Minutes: 10−35 = −25
The minutes are less than 0, so:
- add 60 to Minutes (−25+60 = 60−25 = 35 Minutes)
- and subtract 1 from Hours (3−1 = 2 Hours)
The answer is 2:35
(Notice how we changed "−25+60" to "60−25" ... that is perfectly fine and makes the calculation easier.)
Adjusting The Hours
The hours can also end up too large or small!
To fix that first we must know if it is 24-Hour Clock or AM/PM:
24-Hour Clock
- When Hours end up less than zero: add 24
- When hours end up more than 23: subtract 24
Example: What is 16:20 + 9:35 ?
Add the Hours: 16+9 = 25
Add the Minutes: 20+35 = 55
The minutes are OK
The hours are more than 23, so subtract 24: 25−24 = 1
The answer is 1:55 of the next day.
Example: What is 4:10 − 6:15 ?
Subtract the Hours: 4−6 = −2
Subtract the Minutes: 10−15 = −5
The minutes are less than 0, so:
- add 60 to Minutes: −5+60 = 60−5 = 55 Minutes
- and subtract 1 from Hours: −2−1 = −3 Hours
The hours are less than 0, so add 24: −3+24 = 24−3 = 21
The answer is 21:55 of the previous day.
12-Hour Clock (AM/PM)
For 12-Hour Clock it is best to convert to 24-Hour Clock, do the calculations, then convert back.
Example: What is 3:20 PM − 16:05 ?
3:20 PM is 15:20
Subtract the Hours: 15−16 = −1
Subtract the Minutes: 20−5 = 15
The minutes are OK
The hours are less than 1, so add 24: −1+24 = 24−1 = 23
The answer is 23:15, which is 11:15 PM of the previous day.