Measuring an Exact Span
A duration between two moments is a subtraction of timestamps. The difficulty is not the arithmetic but the mixed-radix units on either side: dates run in months of unequal length, and times run in base 60.
The reliable method is to convert both moments to a single count — seconds since a reference — subtract, then convert the result back into days, hours, minutes and seconds. That is what this calculator does, which is why it handles spans crossing midnight, month ends and leap days without special cases.
Duration Unit Conversions
| Unit | Seconds | Minutes | Hours | Days |
|---|---|---|---|---|
| 1 minute | 60 | 1 | 0.0167 | 0.000694 |
| 1 hour | 3,600 | 60 | 1 | 0.0417 |
| 1 day | 86,400 | 1,440 | 24 | 1 |
| 1 week | 604,800 | 10,080 | 168 | 7 |
| 1 month (avg) | 2,629,746 | 43,829 | 730.5 | 30.44 |
| 1 year (avg) | 31,556,952 | 525,949 | 8,765.8 | 365.2425 |
Duration Versus Calendar Difference
These answer different questions and frequently disagree.
| Duration | Calendar difference | |
|---|---|---|
| Measures | Elapsed time, exactly | How the date labels changed |
| 1 Jan 23:00 to 2 Jan 01:00 | 2 hours | 1 day |
| 31 Jan to 1 Mar | Exact hours elapsed | "1 month 1 day" |
| Affected by month length | No | Yes |
Use duration for anything measured — work, travel, cooking, machine runtime. Use calendar difference for anything described — age, notice periods, anniversaries.
Time Formats
The calculator accepts both conventions:
| 24-hour | 12-hour | Meaning |
|---|---|---|
| 00:00 | 12:00 am | Midnight |
| 09:00 | 9:00 am | Morning |
| 12:00 | 12:00 pm | Noon |
| 17:30 | 5:30 pm | Late afternoon |
| 23:59 | 11:59 pm | One minute to midnight |
Midnight and noon are the two points people reliably get wrong. Ante meridiem means before midday, so 12:00 am is midnight and 12:00 pm is noon — which is why aviation, medicine and the military use the 24-hour clock exclusively.
Daylight Saving and Time Zones
This calculator works in plain local clock time and does not adjust for daylight saving transitions. On the day clocks move forward, the calendar day is 23 hours; when they move back, 25. A span crossing a transition therefore differs from the wall-clock arithmetic by an hour.
For durations that must be exact across a transition — flight times, shift payroll, service level agreements — work in UTC, which has no transitions, and convert only for display.
Leap Seconds
Atomic time and the Earth's rotation drift apart, and 27 leap seconds have been inserted since 1972 to keep them aligned. The practice is scheduled to be abandoned by 2035 because the irregular extra second causes more trouble for computer systems than the drift it corrects. Civil timekeeping, including this calculator, ignores them.
Frequently Asked Questions
Can the span cross midnight?
Yes. Because both a date and a time are entered for each end, spans of any length are handled directly.
What if the end is before the start?
The magnitude is reported and the reversal is noted, rather than returning a negative duration.
Why is my result an hour out?
Almost certainly a daylight saving transition inside the span. Clock arithmetic and elapsed time differ by one hour across those days.
How do I get decimal hours for billing?
The hours row gives the decimal figure directly — 2 hours 30 minutes appears as 2.5 hours, which is what payroll and invoicing systems expect.