Date and time tools

Which of these four do you need?

Date questions look alike and split into four 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
A birth date	Years, months, and days of age	Age calculator
Several cities and a working day	The hours everyone shares	Meeting planner

The date calculator and days between dates follow the same rules for month-end overflow and weekends. They do not share everything else. The leap-day setting lives only on the date calculator; days between dates treats February 28 as one full year after February 29. Business days are counted from different ends.

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 can land on a different date, which is why the page states the order.

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.

Days between dates has no leap-day setting. It treats February 28 as one full year after February 29. The date calculator defaults to March 1 in non-leap years and 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 site 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

Is there an age calculator?

Yes. The age calculator gives an exact age in completed years, months and days, on today, a past date or a future one. It also shows total months, weeks and days, the weekday you were born and your next birthday. A February 29 birthday uses March 1 in common years unless you switch it to February 28.

Which convention should I pick for a contract?

Read the clause first. If it says the period runs "through" a date and counts that date, check Include end date in days between dates. The default leaves the end date out.

Do days between dates and the date calculator follow the same rules?

Mostly. They use the same counting rules, so month-end overflow and weekends work the same way. Two things differ. The leap-day setting, March 1 by default or February 28, is in the date calculator but not in days between dates, which treats February 28 as one full year after February 29. Business days are counted from different ends: days between dates counts the earlier date and, unless you check Include end date, leaves out the later one, while the date calculator starts on the day after, so a start date on a weekend or a pasted holiday can give different answers.

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. Business-day mode here takes out Saturdays and Sundays. Paste your holidays and they come out too. The Saturday and Sunday rule can't be switched off, so if your weekend falls on other days, the count won't match your calendar.

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