Time Calculator

Add or subtract hours, minutes, and seconds — free duration arithmetic

Adding or subtracting hours, minutes, and seconds requires carrying and borrowing across the 60-unit boundary between each time unit, the same fundamental concept as carrying and borrowing across the 10-unit boundary in ordinary decimal arithmetic — except time arithmetic uses base 60 for its sub-units, which is where manual time addition and subtraction most often goes wrong.

Why time arithmetic isn't base-10 arithmetic

Ordinary addition carries to the next column when a digit sum reaches 10; time addition instead carries to the next unit (seconds to minutes, minutes to hours) when a sum reaches 60, not 10 — adding minutes that sum to 75, for instance, requires carrying 1 hour and keeping 15 minutes, not treating 75 as a valid minutes value. Subtraction works the same way in reverse: borrowing from the next-larger unit means borrowing 60 (not 10) of the smaller unit. This base-60 carrying and borrowing is the single detail that trips up manual time arithmetic most often, since it's easy to instinctively apply base-10 carrying logic out of habit, producing a result that looks plausible but is actually wrong by whatever the base-10-vs-base-60 discrepancy happens to be.

A worked example

Adding two time durations where the minutes portions sum to more than 60 requires carrying an hour and reducing the minutes by 60 — treating the raw minutes sum as valid without that carry produces an invalid time value (more than 59 minutes) that needs correcting, exactly the kind of error that happens when base-10 carrying habits get applied to a base-60 calculation without adjustment.

How this connects to your other time calculations

For calculating elapsed time between two clock times rather than adding or subtracting a duration, the Time Duration Calculator handles that related but distinct calculation, and for time-card-style calculations specifically involving clock-in/clock-out pairs with break deductions, the Time Card Calculator applies this same base-60 arithmetic to a payroll-specific context.

Common mistakes

Applying base-10 carrying logic (carrying when a sum reaches 10, or borrowing 10 during subtraction) instead of base-60 logic is the most common time-arithmetic error — always carry or borrow in units of 60 for both the seconds-to-minutes and minutes-to-hours boundaries, not the base-10 habit that's instinctive from ordinary number arithmetic.