Date Duration Calculator
Three date tools in one — the exact duration between two dates, add or subtract days/weeks/months/years, and count Monday–Friday business days. Timezone-safe calendar math with no daylight-saving off-by-one, live in your browser.
Nothing leaves your browser. Every date is handled with timezone-safe calendar math on your own device — no network requests, no account, works offline.
Three questions people ask a date calculator
Almost every date problem is one of three things, so this page does all three side by side. Duration answers “how long between these two dates?” in both a human breakdown (years, months and days) and running totals (days, weeks, hours). Add / subtract answers “what date is 90 days — or 3 months — from here?” and moves in either direction. Business days answers “how many working days are in this range?” by counting Monday to Friday. All three recompute the instant you change any field; there is no button to hunt for.
The include-end-date toggle, and why it matters
This is the number-one reason two date calculators disagree. Counting from 3 June to 5 June gives 2 if you mean elapsed time (two nights in a hotel, two days until a deadline) and 3 if you mean the days themselves (a festival that runs “the 3rd through the 5th” touches three calendar days). Neither is wrong — they answer different questions — so instead of quietly picking one, the duration mode gives you a checkbox and states which it used. The gap is always exactly one day. Leave it off for “time between”; turn it on for “count the days inclusive”.
Timezone-safe calendar math: no daylight-saving off-by-one
A date like 2024-03-10 is a whole calendar day, not a moment in time, and this tool keeps it that way. Every date is pinned to midnight UTC and all the arithmetic — day differences, month adds, weekday tests — reads the UTC parts of that date. UTC has no daylight-saving jumps, so “add one day” is always exactly +86,400,000 milliseconds → the next calendar date, and a span that crosses a spring-forward or fall-back boundary counts whole days cleanly. A calculator built on your local clock can slip: a date sitting near midnight in a DST zone can round onto the previous or next calendar day around the switch, and you get an answer that is one day off for no visible reason. Doing the math in UTC removes that entire class of bug — it is called out in a comment at the top of this tool's code.
Adding months: the clamping rule
Months are different lengths, so “one month after 31 January” is genuinely ambiguous — there is no 31 February. This tool clamps to the last valid day: 31 January + 1 month = 28 February, or 29 February in a leap year. Subtraction mirrors it: 31 March − 1 month = 28/29 February, not 2 March. That is the convention nearly every date library follows, but it has a consequence worth knowing — adding one month twice is not always the same as adding two months, because the first clamp can lose a day or two. Adding days or weeks, by contrast, is exact and never clamps.
Leap years and 29 February
Day counts use the true length of every month, so leap days are neither dropped nor double-counted. From 29 February 2020 to 29 February 2024 is exactly 4 years, 0 months, 0 days — and 1,461 total days, the extra one being the leap day of 2024. Across a non-leap year the same calendar gap is a day shorter. Because the arithmetic walks the real calendar rather than assuming a flat 30- or 365-day period, February with 28 or 29 days is always handled correctly.
Business days — and the honest limit
The business-days mode counts weekdays, Monday through Friday, in the inclusive range from your start date to your end date, both ends included when they fall on a weekday. A single weekend (Friday to the following Monday) is 2 business days; a Monday-to-Friday week is 5. What it deliberately does not do is subtract public holidays. Holidays differ by country, region and even year, and a hard-coded table that is wrong for your location is worse than none at all — so this tool counts weekends out only and says so plainly. If you need to exclude specific holidays, take the count here and subtract them yourself. It is honest by omission rather than confidently wrong.
Reversed ranges and negative results
If you put the later date first, the duration mode still answers — as an absolute distance, so the years, months and days are never negative — and it notes that the range was reversed, so you know the direction flipped, not the size. If you want a genuinely signed answer (“what date is 40 days before this one?”), the add/subtract mode is the right tool: choose Subtract and read the earlier date directly.