TimeCalcy uses one shared engine for its three calculators. It converts each duration into a whole number of seconds, does the arithmetic, and converts the answer back into hours, minutes and seconds. Decimal hours are calculated only for display; they never replace the exact time value.
Add and subtract durations
Multiply hours by 3,600 and minutes by 60, then add seconds. For 1 hour 45 minutes plus 30 minutes, the total is 6,300 + 1,800 = 8,100 seconds. That is 2 hours 15 minutes. Minutes and seconds greater than 59 carry into the next unit. Subtracting 2 hours from 1 hour produces minus 1 hour, not 23:00.
Between clock times
On the same day, subtract the start time from the end time. For an end time on a later day, add 86,400 seconds for each full day after the start before subtracting. 11:50 PM to 12:20 AM one day later is 30 minutes. Equal start and end times are zero at 0 days, 24 hours at 1 day, 48 hours at 2 days, and so on.
Breaks
Breaks are durations, not times of day. A 9:00 AM to 5:30 PM period is 8 hours 30 minutes. Deducting a 30-minute break leaves 8 hours. A break longer than the whole period is rejected instead of returning misleading negative work time.
Decimal hours
1 hour 30 minutes is 1 + 30/60 = 1.5 hours. It is not 1 hour 50 minutes. 1 hour 20 minutes is 1.3333 decimal hours when rounded to four decimal places. Exact seconds remain available beside the rounded display. Rounding is to the nearest last displayed decimal; halfway cases round away from zero.
Limits
Each duration field accepts a whole number from 0 to 999,999,999. Blank individual units count as zero, but a completely empty required duration is rejected. The sum calculator supports up to 100 entries. Negative input components and fractional components are rejected; subtraction may produce a negative result.
Clock inputs support hours:minutes and optional :seconds. Minutes and seconds must be 00–59. A 24-hour clock accepts 00–23 hours; a 12-hour clock accepts 1–12 with AM or PM. The Days after start value is a whole number from 0 to 999,999,999, so multi-day spans are explicit rather than guessed.
Not timezone-aware
These are ordinary clock calculations, not date, timezone, daylight-saving or leap-second calculations. A day is always treated as 24 hours. For a clock change or travel across timezones, these tools do not establish the actual elapsed time.
This is not a payroll, tax, billing-policy or legal-advice tool. Check any important result against its source inputs.