Time Calculator

Time calculator — add and subtract time

Doing arithmetic with time is fiddly because it isn't base ten: sixty seconds make a minute and sixty minutes make an hour, so "2h 45m plus 1h 30m" isn't something you can add column by column. This calculator handles the carrying for you. Add and subtract durations in hours, minutes, and seconds, or add a length of time to a clock time and see when you'll finish — all in your browser, with nothing uploaded.

It updates live as you type and works entirely on your device, so it responds instantly and your figures never leave your phone or computer.

Adding and subtracting durations

The first mode is pure duration arithmetic. Each row is a length of time — hours, minutes, and seconds — with a + or − in front of it, and the calculator totals them. So 2h 30m + 1h 45m gives 4h 15m: the 75 minutes correctly roll over into an extra hour, which is exactly the step that trips people up doing it by hand. You can add as many rows as you need, which is handy for totalling several tasks, laps, or session lengths.

Subtraction works the same way — just switch a row to −. If what you take away is larger than what you've added, the result is simply shown as negative (for example −1h 30m) rather than hidden or clamped, because a negative difference is often the answer you actually wanted, such as how far under a target time you came in.

Adding a duration to a time of day

The second mode answers a different question: "if I start at this time and it takes this long, when will it be done?" Enter a time of day, choose add or subtract, and give a duration. Starting at 14:30 and adding 3h 45m lands at 18:15. It also handles crossing midnight: 22:00 plus 5 hours is 03:00 the next day, and the calculator says so by showing the day offset, so you're never left wondering whether the result is today or tomorrow. Subtracting past midnight rolls back to the previous day in the same clear way.

How it differs from an hours calculator

It's worth being clear about which tool you need. An Hours Calculator works out the gap between two clock times — you give it a start and an end (say 9:00 to 17:30) and it tells you the elapsed time. This time calculator does the opposite kind of job: it does arithmetic with durations you already have, adding and subtracting them, or shifting a clock time forward or backward by a given amount. If you want the time between a start and finish instead, that's a different calculation.

Decimal hours and total minutes

Alongside the hours-minutes-seconds result, the duration mode also shows the total as decimal hours and as total minutes. Decimal hours are what payroll and billing usually want — 4h 15m is 4.25 hours — while total minutes are handy for timers and cooking. Because a duration is really just a count you can scale, if you then need to apply a rate or a percentage to it, the Percentage Calculator can take it from there, also in your browser.

Works on your phone

The calculator is built mobile-first. The hour, minute, and second fields open the numeric keypad, the + / − controls are large enough to tap without zooming, and the time-of-day field uses the native picker. The result appears right below the inputs, and since everything is computed locally it responds instantly with no connection needed.

Privacy: your numbers never leave your device

Every value you enter is used only to do the arithmetic in your browser. Nothing is uploaded, nothing is stored on a server, and there is no account to create. When you close the page the numbers are gone, because they were never sent anywhere in the first place.

Frequently asked questions

How do I add and subtract time?

Enter each duration in hours, minutes, and seconds, and set a + or − in front of it. The calculator totals them and rolls the seconds and minutes over correctly, so 2h 45m + 1h 30m becomes 4h 15m.

Can it add time to a time of day?

Yes. Switch to the time-of-day mode, enter a clock time, choose add or subtract, and give a duration. It handles crossing midnight and tells you the day offset, so 22:00 + 5h shows 03:00 the next day.

What happens if the result is negative?

In duration mode, if the time you subtract is larger than the time you add, the total is shown as a negative duration (like −1h 30m) rather than being hidden — often that difference is exactly what you were after.

Is my data uploaded to a server?

No. Everything runs in your browser — the numbers you enter never leave your device, and nothing is stored or sent anywhere.