# Duration calculator

> How much time is that? Find the time between two dates and clock times, add or subtract a duration, or total a list of hh:mm:ss times.

Free duration calculator: the time between two dates and times, a moment plus or minus hours and minutes, or a total of hh:mm:ss durations.

- Page: https://how.box/datetime/duration/
- Runs entirely in the browser.

## Inputs

Answer link: https://how.box/datetime/duration/?m={m}&sd={sd}&st={st}&ed={ed}&et={et}&dy={dy}&hr={hr}&mi={mi}&se={se}&dir={dir}&l={l}

- `m` (between, add or sum): between (time from a start to an end), add (a moment plus or minus an amount) or sum (total of a list). Example: `add`.
- `sd` (text): Start date, YYYY-MM-DD (between and add). Example: `2027-03-05`.
- `st` (text): Start time, H:MM or HH:MM:SS on the 24-hour clock (between and add). Example: `08:15`.
- `ed` (text): End date, YYYY-MM-DD (between). Example: `2027-03-06`.
- `et` (text): End time, H:MM or HH:MM:SS (between). Example: `12:00:30`.
- `dy` (number 0 to 1e9): Days to add or subtract (add). Example: `3`.
- `hr` (number 0 to 1e9): Hours to add or subtract (add). Example: `5`.
- `mi` (number 0 to 1e9): Minutes to add or subtract (add). Example: `45`.
- `se` (number 0 to 1e9): Seconds to add or subtract (add). Example: `20`.
- `dir` (add or subtract): add or subtract the amount (add). Example: `subtract`.
- `l` (text): Durations to total, separated by spaces, commas, semicolons or "+" (sum). Example: `0:30 1:15:40`.

Example answer: https://how.box/datetime/duration/?m=add&sd=2027-03-05&st=08%3A15&ed=2027-03-06&et=12%3A00%3A30&dy=3&hr=5&mi=45&se=20&dir=subtract&l=0%3A30+1%3A15%3A40

A complete link carries the primary answer in the page HTML; a missing or invalid value shows the default page.

## Agent tools

- `duration_calculate`: Works with lengths of time, with no time zone (every day has 86400 s; daylight-saving changes are not counted).
  - MCP, one POST to https://how.box/mcp: `{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"duration_calculate","arguments":{"mode":"add","start_date":"2027-03-05","start_time":"08:15","end_date":"2027-03-06","end_time":"12:00:30","days":3,"hours":5,"minutes":45,"seconds":20,"direction":"subtract","list":"0:30 1:15:40"}}}`
  - GET: https://how.box/mcp/run/duration_calculate?mode=add&start_date=2027-03-05&start_time=08%3A15&end_date=2027-03-06&end_time=12%3A00%3A30&days=3&hours=5&minutes=45&seconds=20&direction=subtract&list=0%3A30+1%3A15%3A40

## How it works

Time is counted in a mixed scale: 60 seconds make a minute, 60 minutes an hour and 24 hours a day. This tool turns every moment into a count of seconds, does the plain subtraction or addition and turns the result back into days, hours, minutes and seconds. A date is first counted in whole days along the calendar, so months of different lengths and leap years are handled for you.

To total durations such as 1:30:00 and 0:45:30, turn each into seconds (1 h = 3600 s, 1 min = 60 s): 5400 s and 2730 s add up to 8130 s, which is 2 h 15 min 30 s. The hours may go past 24, so a long list reads 26:05:00 rather than being wrapped into days.

Dates and times here have no time zone. Every day is exactly 24 hours, so a clock change for daylight saving time between the two moments is not counted, and the same goes for leap seconds.

## FAQ

**How do I add hours and minutes together?**
Choose Total a list and type each duration as hours:minutes, for example 1:45 2:30. The total is 4:15.

**How do I find how long a shift lasted?**
Choose Time between, enter the start and end date and time. A night shift that crosses midnight just needs the next day's date for the end.

**Does it count daylight saving time?**
No. It works on clock times without a time zone, so a day always has 24 hours. If a clock change falls between your two moments, the real elapsed time differs by an hour.

**Can I subtract a duration from a date?**
Yes. Choose Add or subtract, pick Subtract and enter the days, hours, minutes and seconds.

**Can I enter decimal hours in the list?**
No. Use hours:minutes or hours:minutes:seconds; use the decimal-hours converter for decimal values.

More: [HowToolbox index](https://how.box/llms.txt) · [agent guide](https://how.box/agents.md) · [agent terms](https://how.box/about/agents/)