Adding and Subtracting Time Across Hours and Minutes Correctly
Adding or subtracting times requires borrowing in base 60, not base 10 — when adding minutes produces 60 or more, subtract 60 and carry 1 to the hours; when subtracting minutes goes negative, borrow 60 minutes from the hours column first. Treating time math like ordinary base-10 arithmetic is the most common source of errors.
Time math looks like ordinary addition and subtraction, but the base-60 minute system trips up a manual calculation in a specific, predictable way.
A worked addition example
Adding 2 hours 45 minutes and 1 hour 30 minutes: minutes first, 45 + 30 = 75, which is 60 or more, so subtract 60 (leaving 15 minutes) and carry 1 to the hours column. Hours: 2 + 1 + 1 (carried) = 4. Total: 4 hours 15 minutes — not "3 hours 75 minutes," which isn't a valid time expression.
A worked subtraction example
Subtracting 1 hour 45 minutes from 3 hours 20 minutes: minutes first, 20 − 45 is negative, so borrow 60 minutes from the hours column, making it 80 − 45 = 35 minutes, and reduce the hours column by 1 (from 3 to 2) to account for the borrow. Hours: 2 − 1 = 1. Total: 1 hour 35 minutes.
Why this is such a common source of errors
Base-10 arithmetic habits are so automatic that it's easy to add or subtract minutes as if 100 minutes made an hour instead of 60 — this single substitution is responsible for the majority of manual time-math mistakes, especially when working quickly or across several time entries at once.