Free online tool
Time Duration Calculator
Pick a start and end date/time and see the exact duration between them.
Why measure exact durations?
Freelancers measure durations to bill clients. Runners measure them to track pace. Project managers measure them to compare estimated vs actual time on task. Doing the math by hand — especially across days, months, or timezones — is error-prone. A calculator eliminates the guesswork.
How this tool works
Pick a start and end date and time using your browser's built-in picker. The tool computes the difference as an exact number of milliseconds and breaks it down into days, hours, minutes, and seconds, plus handy totals for hourly billing.
+Does it handle daylight saving time?
Times are interpreted in your local timezone. Around DST changes, the total may be off by an hour compared to a pure UTC calculation.
+Can I go backwards in time?
Yes — the result is always the absolute difference, so end before start gives the same answer as start before end.
Measuring the gap between two dates without getting fooled by the clock
Working out 'how long between these two moments' seems like plain subtraction, but daylight saving transitions, varying month lengths and timezone differences all quietly distort the result if you're not accounting for them.
Most duration mistakes come from treating the calendar like a uniform grid when it really isn't, and the cases below are the ones people run into most often.
The daylight saving trap
If you calculate the duration between 1:00 AM and 4:00 AM on a day when clocks spring forward, the clock-face difference is 3 hours but only 2 hours actually elapsed, because one hour was skipped entirely. Conversely, on a fall-back night the same clock-face gap can represent one extra real hour. Calculations done in UTC avoid this problem; calculations done on local wall-clock time can silently absorb or lose an hour.
Months and days don't divide evenly
From 31 January to 3 March isn't a clean 'one month and a few days' because February can have 28 or 29 days. The calculator counts actual calendar days elapsed rather than assuming a fixed 30-day month, which is why a duration expressed as '1 month, 3 days' can represent a different number of raw days depending on which months it spans.
Worked example
From 10:00 AM on 1 June 2024 to 3:30 PM on 15 September 2024 is 106 days and 5.5 hours, or roughly 3 months and 14 days — a case worth double-checking manually if precision to the hour genuinely matters, since 2024 being a leap year doesn't affect this particular span but would shift things by a day if the range crossed 29 February.
Timezone differences compound the problem
Comparing a start time in New York with an end time in Tokyo requires converting both into the same reference frame before subtracting, and forgetting that step can throw a result off by up to a full day depending on the direction of travel across the date line. Project deadlines and flight-duration calculations are the two most common places this bites people, since a flight departing at 11 PM local time and landing at 6 AM local time the next day isn't necessarily a 7-hour flight once timezone offsets are untangled.
Practical uses beyond curiosity
This kind of duration math underlies contract terms ('net 30 days from invoice date'), project timelines, rental agreement lengths, warranty expiration dates, and even calculating someone's exact tenure at a job for a resume — all situations where 'about a month' isn't precise enough and the actual day count matters.
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