This page documents the formulas, input limits, and checks used by the calculators. The goal is to make a result easy to reproduce.
Durations and clock times
A duration is stored in whole seconds. For each signed row, seconds = 86,400 × days + 3,600 × hours + 60 × minutes + seconds. The homepage sums signed rows and splits the magnitude into days, hours, minutes, and seconds.
To shift a known clock time, add or subtract the duration from seconds since midnight. Divide by 86,400 and round down to get the day offset. The remainder gives the displayed clock time. Elapsed time subtracts start from end and adds a day only when the next-day option is selected.
Time units follow the NIST guide to time units. These clock calculations use uniform days and do not account for time zones, daylight saving, or leap seconds.
Decimal hours and weekly work time
Decimal hours are total seconds divided by 3,600, displayed to two decimal places. Reverse conversion multiplies by 3,600 and rounds half up to the nearest second. The time card subtracts unpaid break minutes from each valid shift and sums exact net minutes before displaying decimal hours. It does not apply payroll or overtime rules.
Calendar dates
The date calculators validate dates using the Gregorian calendar, from 1900 through 9999. They count date ordinals, so a local daylight saving change does not add or subtract a calendar day. The inclusive count adds one day to the elapsed result. Day offsets include weekends and holidays. See the U.S. Naval Observatory leap year rule.
Speaking time
Estimated minutes equal word count divided by words per minute. The main result is rounded to a second. A measured rehearsal is a better guide when pauses and delivery vary.
Checks and version history
November 9, 2026. The homepage and supporting tools use explicit validation and sample inputs. Checks cover overnight times, zero results, decimal precision, breaks, Gregorian leap dates, and invalid entries. Each calculator’s example gives values you can enter to reproduce the result.
To report a discrepancy, use Contact and include the page URL, sample values, what you expected, and what the calculator returned.