How Time Addition and Subtraction Works
Time addition shifts a clock time forward by a given duration; subtraction shifts it backward. The result is normalized across the 24-hour day, and a day offset is calculated to show whether the result falls on the same day, the next day, or an earlier day.
The formula converts all inputs to total seconds: result_seconds = start_seconds ± delta_seconds. The day offset is floor(result_seconds / 86400). The time of day is extracted from the remainder.
Example: adding 8 hours to 20:00:00 gives 72000 + 28800 = 100800 total seconds. Day offset = 1 (next day). Time of day = 100800 − 86400 = 14400 seconds = 04:00:00.
Duration Between Two Times
Without dates, an end time earlier than the start is interpreted as crossing midnight. For example, 23:00 to 01:30 produces 2 hours 30 minutes, not a negative result. This handles most shift-work and event-timing cases automatically.
For spans longer than 24 hours, enable the date fields. Calendar dates are combined with clock times and the total difference is calculated in UTC to avoid daylight-saving discontinuities. If the end date/time is before the start, an error is shown.
Time Unit Conversion
Convert mode accepts decimal inputs. Enter 1.5 hours and select Hours: the result shows 1 hour 30 minutes, totalling 5,400 seconds and 90 minutes. The calculation rounds to the nearest second, so very small fractions at high unit values may lose precision.
Common conversions: 1 day = 24 hours = 1,440 minutes = 86,400 seconds. 1 hour = 60 minutes = 3,600 seconds. 1 minute = 60 seconds.
Timezone and DST Assumptions
This calculator is entirely timezone-free. Clock times are treated as abstract 24-hour values with no timezone or daylight-saving offset. For calculations spanning a DST transition, add or subtract the clock-change amount (usually 1 hour) manually. For cross-timezone work, convert both times to UTC first.