Time Calculator

The time calculator does three things in one place: it adds and subtracts hours, minutes, and seconds across as many rows as you need; it works out the exact difference between two clock times; and it converts a duration between seconds, minutes, hours, days, and weeks. Instead of counting on your fingers or fighting with a spreadsheet's time formats, you type the values and read off the answer in plain hours-minutes-seconds as well as in decimal hours.

Who it is for

It suits anyone who works with durations — a freelancer totalling billable hours, an employee adding up a week's timesheet, a student timing study sessions, a runner or cyclist summing lap times, or a project manager working out elapsed time between two events. Because it shows both the human-readable format and the decimal-hour equivalent, it bridges the gap between how we say time and how payroll or billing systems expect it.

It is equally useful for everyday planning: working out how long a journey will take from departure and arrival times, totalling the runtime of a playlist or a set of video clips, planning cooking and baking stages, or checking whether back-to-back meetings actually fit into an afternoon. Anywhere you would otherwise reach for pen and paper to carry minutes over into hours, this calculator does the carrying for you and removes the small arithmetic slips that creep in by hand.

How it works — the method

Internally every entry is normalised to a single base unit before any arithmetic happens, which is the reliable way to handle the fact that minutes and seconds wrap at 60, not 100. Add / Subtract mode converts each row to total seconds, applies its plus or minus sign, sums everything, and then converts the result back into hours, minutes, and seconds — so 1h 30m + 0h 45m correctly becomes 2h 15m, not 1h 75m. A negative total (when you subtract more than you have) is shown as a negative duration.

Time Difference mode takes two clock times, converts each to minutes from midnight, and reports the absolute gap between them — useful for shift lengths or “how long until” calculations. Convert Units mode uses the fixed relationships below: it turns your input into seconds first, then divides to express it in every other unit at once.

Time unit conversions

1 minute = 60 seconds  ·  1 hour = 60 minutes = 3,600 seconds  ·  1 day = 24 hours = 1,440 minutes  ·  1 week = 7 days = 168 hours

Worked example

Say you log three work sessions in a day: 2h 30m, 1h 45m, and 0h 50m. Adding them, the calculator converts to seconds (9,000 + 6,300 + 3,000 = 18,300 seconds), sums, and converts back to 5h 5m 0s, also shown as roughly 5.0833 decimal hours. If your rate is set per decimal hour, that 5.0833 figure is the number you would multiply — which is exactly why simply writing “5h 5m” into a payroll cell as 5.05 would underpay you.

Tips

Use the decimal-hour output for any billing or payroll work, since those systems almost never accept “5h 5m” directly. In Add / Subtract mode you can stack as many rows as you like and flip any row to subtraction, which is handy for deducting breaks from a shift. For two specific clock times, the Difference mode is faster and avoids manual carrying.

Common mistakes

The classic error is treating minutes as hundredths — writing 1h 30m as 1.30 hours instead of the correct 1.5 hours. Another is forgetting that subtracting a larger duration yields a negative result, which is valid here, not an error. When two clock times span midnight, the difference mode shows the absolute gap; if the end time is earlier than the start time and the period actually crosses midnight, add 24 hours to the displayed result.