Date and time tools
Start with days between dates for a span, or the date calculator to add or subtract days, weeks, months, or years from one start date. Both tools publish how they count and keep your inputs in the URL fragment only. The meeting planner remains for multi-city overlap.
Every tool in this cluster
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Meeting planner
Shows one day grid for several cities, marks work-hour overlap, applies daylight saving from IANA zones, and shares the plan only in the URL fragment.
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Days between dates
Counts calendar days between two dates in the browser with no account, and shares the range only in the URL fragment.
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Date calculator
Adds or subtracts a day count from a start date in the browser, with the result and inputs shareable in the URL fragment only.
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Age calculator
Walks forward from the birth date so the day count can never go negative, states the constrain and leap-day rules on the page, and shows the leap-day control only for a 29 February birth date.
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Does the end date count
Separates the difference from the span as two different questions, names which convention deadlines, stays, payroll and terms each use, and points at the tool control that switches between them instead of asking for mental arithmetic.
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What 5 to 7 business days means
Works a Monday order through cut-off, weekends and day one to an eleven calendar day answer, and separates warehouse processing from transit as two clocks that are usually quoted as one.
Which of these three do you need?
Date questions look alike and split into three jobs. Pick by the shape of what you already know.
| You know | You want | Tool |
|---|---|---|
| Two dates | The gap between them | Days between dates |
| One date and a length of time | The date you land on | Date calculator |
| Several cities and a working day | The hours everyone shares | Meeting planner |
The first two run on the same engine, so they agree with each other by construction. That matters more than it sounds, and the rest of this page explains why.
Do you count both the start day and the end day?
This is the question that makes two calculators disagree, and a result box on its own can't tell you which of the two answers you're looking at.
A contract that runs "from January 1 through January 31" usually means thirty-one days. An engineer measuring January 1 to January 31 means thirty. Same two dates. Different number. Neither is wrong, because they're answering different questions.
Days between dates counts exclusive by default: from the start date up to but not including the end date. Check Include end date and it adjusts by one day in the direction of the span. The page shows you which convention produced the number, on the page, next to the number.
Reversed ranges follow from that rather than being a special case. If inclusive 1 January to 10 January is 10 days, then inclusive 10 January to 1 January is -10 days. Not -8. The sign flips and the magnitude holds.
Why does January 31 plus one month cause trouble?
February doesn't have a 31st, so something has to give. Two reasonable answers exist: slide forward into March, or stop at the last day February has.
Both tools use constrain overflow. January 31 plus one month is February 28, or February 29 in a leap year. It isn't March 2 or March 3. If you've ever watched a billing date drift a few days later every spring, you've seen the other choice at work.
Order matters too. When a calculation mixes units, the date calculator applies years, then months, then days. Run those in a different order and you land on a different date for real inputs, which is why the rule is written down rather than left to the implementation.
What happens to a February 29 birthday?
Leaplings get an anniversary three years out of four, and the calendar has no opinion about which day it should be. Jurisdictions do, and they disagree. UK law treats anniversaries differently from some US practice.
The shared date engine defaults to March 1 in non-leap years. The date calculator carries a toggle for February 28 if that's the convention you need. We disclose the choice instead of burying it, because a leap-day answer without a stated rule is a guess wearing a result box.
Multi-year jumps skip the question entirely. February 29, 2024 plus four years is February 29, 2028, which is a leap year, so no anniversary rule applies at all.
Does daylight saving change a day count?
No, and the reason is worth a sentence. Calendar math here never steps in 86,400,000 millisecond chunks. It works on calendar dates directly.
Clock-based date math is where the classic off-by-one comes from: two days in March turn into 47 hours, the arithmetic rounds down, and a span quietly loses a day. Working on dates instead of milliseconds means a clock change can't reach the answer.
Time zones are a different problem, and that's the one the meeting planner handles. It builds a single day grid, marks where working hours actually overlap, and applies daylight saving from each IANA zone using your browser's own time zone data.
Business days, and what we left out on purpose
Both calculators can count business days. The scope is the same on each: weekends come
out, and so does any date you paste into the custom holidays box, one
YYYY-MM-DD per line.
Country holiday calendars aren't built in. They need ongoing maintenance this repository doesn't carry, and a stale holiday list is worse than no holiday list, because it fails silently and looks authoritative while it does. Paste your own and you know exactly what was excluded.
Where do your dates go?
Nowhere. The dates you type stay in the tab. Share links carry state after the hash mark, which browsers don't send to a server, so a link you paste into chat reconstructs the inputs on the other person's machine without them passing through ours. Once a page has loaded, you can turn your wifi off and keep working.
Each page also shows its own measured request count and script weight while you use it. You don't have to take the claim on faith. More on the method in how we build tools.
Questions
Which convention should I pick for a contract?
Legal and contract counts usually want inclusive, because "through" a date normally includes it. Check Include end date. When it matters, read the clause rather than trusting a default, including ours.
Why do the two calculators always agree?
They share one engine. The counting rules, the overflow behavior and the leap-day default live in a single place, so a change lands on both pages at once and they can't drift apart.
Can I count backwards?
Yes, both ways. Put the later date first in days between dates and you get a signed result. Or pick Subtract in the date calculator, where direction comes from the control alone and negative amounts are refused, so the switch and the number can never contradict each other.
Does "business day" mean the same thing everywhere?
No, and that's the reason the holiday box is empty rather than pre-filled. Business-day mode here excludes Saturdays and Sundays, which is the common pattern but not a universal one. Weekends fall on different days in parts of the Middle East, and public holidays vary by country, by state, and sometimes by year. Paste the dates that apply to you and the exclusion list is exactly as accurate as your own knowledge of the calendar.
Is there an age calculator?
Not yet. It's specced and it's next in this cluster. Until it ships, the date calculator handles the same arithmetic if you supply the two dates yourself.
Related classroom tools
Scheduling a class is a different job from scheduling a remote standup. For fair name picking, groups, and seating, use the classroom tools. Material changes land in the changelog.