Time duration calculator
The atomic second — defined in 1967 as exactly 9,192,631,770 oscillations of a cesium-133 atom — replaced astronomy-based timekeeping because Earth's rotation varies. We gain or lose roughly 2 milliseconds per century due to tidal friction, which is why UTC occasionally adds leap seconds. Your 8-hour workday is 28,800 of these cesium-calibrated seconds, precise to within a few nanoseconds.
Good to know
Decimal hours are the payroll standard. Most timesheet systems record hours as decimals: 7:30 is 7.5 hours, 7:45 is 7.75 hours. This makes wage calculations straightforward — $20/hour × 7.5 hours = $150, no fraction conversion needed. We show both formats because some contexts use hours:minutes while others need decimals.
24-hour time exists to prevent deaths. Hospitals, military, and aviation use 24-hour format because AM/PM confusion has proven fatal. A medication scheduled for "6:00" should never be ambiguous. There's no 6:00 AM/PM distinction in 24-hour time — it's 06:00 or 18:00. The aviation industry adopted it after scheduling errors contributed to crashes.
Time zones require separate conversion. This calculator assumes both times are in the same zone. A flight departing LAX at 10:00 PST and arriving JFK at 18:00 EST isn't an 8-hour flight — it's 5 hours (10:00 PST = 13:00 EST). For cross-timezone calculations, convert all times to a common zone first.