How do I calculate a duration that spans past midnight?

The 'Duration Between Times' calculator automatically handles this. If the End Time is earlier than the Start Time (e.g., Start: 10:00 PM, End: 2:00 AM), it assumes the End Time is on the following day and adds 24 hours to the calculation.

What format should I use for time?

For the 'Duration Between Times' tab, use the 24-hour time picker provided by your browser (HH:MM:SS). For the 'Add/Subtract Laps' tab, enter positive numbers for hours, minutes, and seconds.

Can I use this as an actual stopwatch?

This is a 'stopwatch calculator' for time arithmetic, not a real-time stopwatch. To time an event, you would use a stopwatch (like on your phone) and then enter the start/end times or lap times here to perform calculations and find the total.

What is sexagesimal (base-60) arithmetic?

It's the numbering system we use for time, where each unit is a multiple of 60 (60 seconds in a minute, 60 minutes in an hour). This is different from the decimal (base-10) system we use for regular math. This calculator handles all base-60 conversions automatically.

How precise are the calculations?

The calculations are mathematically exact to the second. The tool does not currently support milliseconds or fractions of a second.

Stopwatch Calculator - Calculate Time Durations & Laps

A precise stopwatch calculator to find the time duration between a start and end time, or to add and subtract multiple time durations (laps).

Full original guide (expanded)

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[", right: "\]

", right: "

Formula (extracted text)

$ T_{\text{seconds}} = (\text{hours} \times 3600) + (\text{minutes} \times 60) + \text{seconds} $

Formula (extracted text)

$ \Delta T = (T_{end, \text{seconds}} - T_{start, \text{seconds}}) $

Formula (extracted text)

$ \text{If } \Delta T < 0, \text{ then } \Delta T_{\text{final}} = \Delta T + 86400 $

Formula (extracted text)

$ T_{total} = \sum_{i=1}^{n} (\pm t_{i, \text{seconds}}) $

Formula (extracted text)

$ \text{Hours} = \lfloor \frac{T_{total}}{3600} \rfloor $ $ \text{Minutes} = \lfloor \frac{T_{total} \mod 3600}{60} \rfloor $ $ \text{Seconds} = T_{total} \mod 60 $

Variables and units

No variables provided in audit spec.

Sources (authoritative):

iso.org/standard/70907.html — iso.org · Accessed 2026-01-19
https://www.iso.org/standard/70907.html
Home — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/
Everyday Life — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/lifestyle-everyday
Time and Date — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/subcategories/time-date
Time Calculator (Add/Subtract) — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/time-calculator
Date Calculator (Days Between) — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/date-calculator

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
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Ad Placement (300×600)

Calculator Features

Find elapsed time between any start and end time.
Automatically handles midnight-spanning calculations.
Add or subtract lists of lap times or project hours.
See results in HH:MM:SS, total hours, total minutes, and total seconds.

Related Time Tools

Time Calculator (Add/Subtract)
Date Calculator (Days Between)
Hours and Minutes Calculator
Project Timeline Calculator

Formulas

(Formulas preserved from original page content, if present.)

Version 0.1.0-draft

Citations

Add authoritative sources relevant to this calculator (standards bodies, manuals, official docs).

Changelog

0.1.0-draft — 2026-01-19: Initial draft (review required).