Date maths that gets leap years and time zones right
Ten calculators for days between dates, ages, business days, durations, week numbers, time zones and countdowns. Calendar arithmetic is full of traps, and doing it in your head is how deadlines get missed by a day.
Counting days sounds trivial until February, a leap year, a public holiday and a time zone all turn up in the same calculation. Add the question of whether the end date counts, and two reasonable people get two different answers to what looks like one subtraction.
These calculators use a proper calendar implementation rather than dividing by 86,400, so leap days, month lengths and daylight saving transitions are handled the way the calendar actually behaves. Each result says explicitly whether the endpoints are included.
No tool matches that.
Date Calculations
- Days Between Dates Calculator
Find the exact number of days, weeks, or months between any two dates.
- Age Calculator
Calculate exact age in years, months, and days from a birth date, plus days until your next birthday.
- Date Add/Subtract Calculator
Add or subtract days, weeks, months, or years from any date to find the resulting date.
- Business Days Calculator
Count working days between two dates, excluding weekends and any holidays you list.
- Leap Year Checker
Check whether any year is a leap year, with the exact rule explained and the next leap years listed.
Time & Scheduling
- Time Zone Converter
Convert a time from one time zone to another, with automatic daylight saving adjustment.
- Time Duration Calculator
Find the exact hours and minutes between two times, including overnight shifts that cross midnight.
- Countdown Timer
Count down to any future date and time, live, down to the second.
- Week Number Calculator
Find the ISO week number for any date, or the date range for any week of the year.
Holidays
- Next Holiday Countdown
See how many days remain until the next public holiday in your country.
From the blog
Which calculation are you doing?
Counting elapsed time? Days between dates and the age calculator handle the same problem at different scales, and both show the answer in years, months and days as well as a raw total, because a contract and a birthday want different phrasings of the same interval.
Working to a deadline? Date add and subtract handles a plain offset, and the business days calculator skips weekends so a five-working-day turnaround lands where a client would expect. The countdown timer turns a target date into a live figure worth putting on a screen.
Coordinating across places? The time zone converter shows the same moment in several zones at once, including the offset changes that make a call scheduled a month out land an hour off. Week number matters if your organisation plans in ISO weeks, where the first week of the year is not always the one containing 1 January.
Why the details matter
Daylight saving is the classic source of quiet errors: two countries switch on different dates, so the gap between them changes twice a year. A converter that ignores it is right most of the year and confidently wrong for a few weeks.
Everything runs in your browser using your device's own time zone database, so no date, deadline or event you type is sent anywhere.
Common questions
Does the day count include both the start and end dates?
The default is the elapsed interval, so a Monday to a Friday is four days. Each calculator also shows the inclusive count, which is five, because contracts and notice periods usually mean that one. The result labels which is which so there is nothing to guess.
Are leap years handled?
Yes, including the century rule that trips up simple implementations: 2000 was a leap year and 1900 was not. The calculation uses real calendar dates rather than a fixed number of seconds per day.
Do the business day calculations skip public holidays?
They skip weekends by default and let you add specific holiday dates, which is the honest approach. A tool that claims to know every public holiday in your country and region will be wrong somewhere, and a wrong holiday is worse than one you entered yourself.
How does the time zone converter handle daylight saving?
It uses your browser's built-in time zone database, so each zone's own switch dates are applied to the date you picked. That is why converting a date in July can give a different offset from the same pair of cities in January.
What is an ISO week number?
The ISO 8601 standard numbers weeks from Monday and assigns week 1 to the week containing the first Thursday of the year. It means early January can fall in week 52 or 53 of the previous year, which surprises people every single January.