Adding and Subtracting Time — Definition, Formula & Examples
Adding and subtracting time means combining or finding the difference between amounts of time measured in hours, minutes, and seconds. Because time uses groups of 60 (not 100), you must regroup carefully when minutes or seconds exceed 60 or drop below 0.
Time arithmetic involves performing addition or subtraction on quantities expressed in a mixed-unit system where 1 hour = 60 minutes and 1 minute = 60 seconds. When a sum of minutes or seconds reaches 60 or more, the excess is carried to the next larger unit; when a subtraction requires borrowing, 1 unit of the next larger denomination is converted into 60 of the smaller.
How It Works
Line up hours under hours, minutes under minutes, and seconds under seconds. Add or subtract each column starting from the smallest unit. If a sum of minutes or seconds is 60 or more, subtract 60 from that column and carry 1 to the next larger column. If you need to subtract a larger number of minutes from a smaller one, borrow 1 hour (which becomes 60 minutes) and add it to the minutes column before subtracting.
Worked Example
Problem: A movie starts at 2:45 PM and lasts 1 hour 30 minutes. What time does it end?
Add the minutes: Add 45 minutes + 30 minutes.
Regroup: Since 75 minutes is more than 60, convert 60 minutes into 1 hour and keep the remaining 15 minutes.
Add the hours: Add the original hours plus the carried hour: 2 + 1 + 1 = 4 hours.
Answer: The movie ends at 4:15 PM.
Why It Matters
You use time arithmetic whenever you plan a schedule, calculate travel duration, or figure out cooking times. It also prepares you for working with other non-decimal measurement systems, such as feet and inches.
Common Mistakes
Mistake: Treating minutes like regular base-10 numbers (e.g., writing 1:75 instead of 2:15).
Correction: Remember that 60 minutes make 1 hour. Whenever minutes reach 60 or more, subtract 60 and add 1 to the hours.