Time Between Two Times
End time is the next day
An hour is defined as exactly 3,600 seconds, making it one of the few everyday time units with a precise SI relationship. The modern hour descends from ancient Babylonian timekeeping, where the day was divided into 12 double-hours, each further split into 30 gesh of two minutes each. When the Greeks adopted this system, they halved the double-hour and arrived at the 24-hour day that virtually every culture still uses. Despite the proliferation of decimal time proposals during the French Revolution — which experimented briefly with a 10-hour day of 100 minutes each — the 24-hour clock proved too entrenched to displace.
In everyday life the hour is most often encountered in two flavors: clock hours, which are fixed 60-minute intervals, and work hours, which vary by jurisdiction. A standard U.S. workweek is 40 hours, but a legal "work hour" for overtime calculations counts fractional hours differently depending on whether an employer rounds to the nearest quarter-hour, the nearest tenth, or not at all. When payroll precision matters, working in total minutes and converting at the end avoids rounding compounding errors — exactly the approach this calculator takes internally.
The 12-hour clock that splits the day into AM (ante meridiem, before midday) and PM (post meridiem, after midday) originated in ancient Egypt and Rome but became standard in English-speaking countries during the industrial era. Its chief disadvantage is ambiguity around midnight and noon: 12:00 AM and 12:00 PM are routinely confused even by native speakers, and scheduling errors around these pivot points are common. The 24-hour clock, sometimes called military time, eliminates the ambiguity entirely — 00:00 is midnight and 12:00 is noon, with no qualifier needed.
Aviation, the military, hospitals, and international rail and air schedules all use 24-hour notation for this reason. The ISO 8601 standard, which governs most digital time representation, also mandates the 24-hour format. When calculating elapsed time across midnight, using 24-hour logic internally and converting back to 12-hour only for display (as this calculator does) is the most reliable approach.
| Time Period | Hours |
|---|---|
| Day | 24 |
| Week | 168 |
| Month (avg) | 730.5 |
| Quarter (avg) | 2,191.5 |
| Year (common) | 8,760 |
| Year (leap) | 8,784 |
| Decade | 87,600 |
| Century | 876,000 |