Time Duration Calculator

The Kordu Time Duration Calculator does two jobs that every payroll clerk, consultant, shift worker, and project manager eventually needs: figuring out how long something took, and figuring out what time it will be after adding or subtracting a duration. Both look trivial on paper and both trip people up in practice — base-60 arithmetic, overnight wraparound, and decimal-hour conversion all have edge cases that silently corrupt timesheets if you do them by hand.

Two modes in one tool

Duration mode takes a start time and an end time in 24-hour HH:MM format and returns the duration in four formats simultaneously: HH:MM (e.g. 8:30), total minutes (510), total seconds (30600), and decimal hours (8.5). The four formats cover every downstream system you might paste into — HH:MM for human-readable reports, minutes or seconds for scripting and spreadsheet math, and decimal hours for payroll, billing rates, and invoice line items.

Add/subtract mode takes a start time plus a duration expressed as hours and minutes, and returns the resulting clock time. Adding 8 hours and 30 minutes to a 07:00 clock-in produces a 15:30 clock-out. Subtracting 45 minutes of prep time from a 14:00 deadline produces a 13:15 start time. The result wraps around midnight correctly in both directions: adding 5 hours to 21:00 produces 02:00 (not an error, not 26:00), and subtracting 3 hours from 01:00 produces 22:00 the previous day.

Why precise time math matters

Payroll is the highest-stakes case. A single shift miscalculated by fifteen minutes, repeated across thirty employees and twenty-six pay periods, compounds into thousands of dollars of payroll error every year. Hourly contractor billing has the same problem — a consultant tracking six concurrent engagements cannot afford the cumulative drift of mental arithmetic across a week of five-minute clock-ins. The same math powers shift scheduling, cooking and baking timers, interval training sessions, study blocks, meeting lengths, fasting windows, medication intervals, and any other task where "how long" or "when will it end" has to be right.

Decimal hours, explained

Clock time is base-60 (sixty minutes to the hour), but almost every payroll, accounting, and project-tracking system multiplies by a flat rate per hour — a base-10 operation. That mismatch is why decimal hours exist. Eight hours and thirty minutes is 8.5 hours because thirty minutes is 0.5 of an hour; seven hours and forty-five minutes is 7.75 hours because forty-five minutes is 0.75 of an hour (45 ÷ 60). Fifteen minutes is 0.25, twenty minutes is 0.333…, ten minutes is 0.1666…. Converting in your head is error-prone, which is why Harvest, Toggl, Clockify, QuickBooks Time, ADP, and essentially every other time-tracking and payroll platform export decimal hours by default. The calculator shows decimal hours alongside HH:MM so you can copy the right format into whichever system you're feeding.

Overnight shift math — why "Crosses midnight" exists

A naïve end − start calculation fails the moment a shift crosses midnight. 22:00 to 06:00 is eight hours of work, but 06:00 − 22:00 as raw numbers is −16 hours. The fix is to add 24 hours to the end time when the shift wraps past midnight, giving 30:00 − 22:00 = 8:00. That's exactly what the "Crosses midnight" toggle does. Flip it on for any range where the end time is numerically earlier than the start time: hospital night shifts (19:00 to 07:00), bar and restaurant close-down (23:30 to 03:00), warehouse graveyards (23:00 to 07:00), long-haul driving legs (21:00 to 05:00), or any weekend DJ set that runs into the small hours. Without the toggle, a midnight-crossing range returns an error rather than a silently wrong negative — the tool refuses to guess on your behalf so you don't accidentally pay someone for negative time.

Add/subtract use cases

Add mode answers "what time will my shift end?" (clock-in + shift length), "when does my Pomodoro finish?" (now + 25 minutes), "what's the deadline if I start now and the task takes 4 hours?", and "when should I take my next dose if the interval is 6 hours?". Subtract mode answers "what time do I need to leave?" (arrival − travel time), "when should I start prep?" (deadline − prep duration), and "what time was 90 minutes ago?". Both modes respect the 24-hour clock, so 00:00 and 23:59 wrap cleanly in either direction.

Payroll rounding conventions

Different jurisdictions and employers round differently. Some round clock-ins to the nearest quarter-hour (7-minute rule: 0–7 minutes round down, 8–14 round up), others use tenth-hour rounding (every six minutes), and many modern systems bill to the exact minute. This calculator does not round — it returns the exact duration so you can apply your employer's or client's rounding rule consciously rather than inheriting whatever a spreadsheet decides. If your contract says "billed in 15-minute increments," take the decimal-hour result and round up to the next 0.25.

24-hour vs 12-hour formats

The tool uses 24-hour HH:MM input to eliminate AM/PM ambiguity — 7:00 is unambiguous as "morning" only with the AM suffix, and at shift-handover time that suffix is exactly where mistakes happen. If your source data is in 12-hour format, convert 1 PM through 11 PM by adding 12 (1 PM = 13:00, 11 PM = 23:00) and keep 12 AM as 00:00 and 12 PM as 12:00.

Edge cases and common mistakes

If the start and end time are equal, the duration is 0:00 — fine for a canceled appointment, suspicious for a logged shift. If the start is later than the end and you forget the midnight toggle, the tool returns a clear error rather than silently producing a negative number. If you're chaining calculations (shift + overtime + travel), do the arithmetic in minutes or decimal hours and convert back to HH:MM at the end — adding HH:MM values directly requires carrying the sixty-minute rollover manually, which is exactly the error the tool exists to prevent.

Spreadsheet time math for comparison

Excel and Google Sheets store dates and times as fractions of a day, so =END-START between two time cells returns a fractional-day value. Formatting the result cell as [h]:mm (square brackets are critical — they tell the spreadsheet to allow durations over 24 hours) gives you HH:MM. Multiplying by 24 gives decimal hours; multiplying by 1440 gives total minutes. The tool gives you all three without the format-cell dance.

Where this fits in a time-tracking workflow

Use this calculator as a sanity-check when reconciling timesheet totals, when a Toggl or Harvest export needs a one-off manual entry, when a client asks "what did I bill you for on the 14th?", or when you need to convert between formats to paste into a payroll system that only accepts decimal hours. It pairs naturally with the Kordu Date Difference tool (for multi-day spans), the Stopwatch and Pomodoro Timer (for live tracking), the Timezone Converter (for cross-region meetings), and the Salary to Hourly Calculator (to turn tracked decimal hours into dollar amounts).

Privacy and data processing

Every calculation runs client-side in your browser using plain JavaScript. Kordu does not log, transmit, or store the times you enter, the durations you compute, or any other data. The tool works offline once the page has loaded and has no rate limits, signup walls, or third-party trackers reading your inputs.