Where date maths goes wrong
Dates look like simple subtraction until a time zone or a leap year gets involved. These are the cases people most often get wrong by hand: converting a meeting time across daylight saving boundaries, counting the days between two dates, and working out an exact age in years, months and days.
- Daylight saving time: the offset between two cities changes twice a year, and not on the same dates. The US switches in March and November, most of Europe in late March and late October, and places like Sydney switch in the opposite direction. The time zone converter applies each city's rule for the date you enter, so the same 9 am call can land an hour apart in spring and autumn.
- Counting inclusively: “days until” usually excludes today, while “days between” for a contract usually includes both ends. Decide which one you need before you paste the number anywhere.
- Leap years and month lengths: an age in “years, months and days” depends on which month you are counting through. The age calculator counts calendar months from the birth date rather than dividing total days by 30, which is why its answer can differ from a quick estimate.
Which tool to open
- Scheduling a call, a webinar or a livestream with people in other countries: time zone converter. Enter the date as well as the time so daylight saving is handled.
- Filling in a form that asks for an exact age, checking an age limit, or counting down to a birthday: age calculator.
- Days between two dates, or the date some days from now: date calculator.
- How long until an event, updating every second: countdown timer.
- Reading or writing 24-hour time such as 1730: military time converter.
After you have the number
When you paste a converted time into an invite or a message, include both cities (for example “9:00 New York / 14:00 London”) so nobody has to convert it again.
Common questions
Why did my recurring meeting move by an hour?
One side crossed a daylight saving boundary and the other did not, or they crossed on different dates. The US changes in March and November, most of Europe in late March and late October, and the southern hemisphere in the opposite direction. Convert the time for the specific date rather than relying on a fixed offset. Time zone converter
Does the converter handle daylight saving automatically?
Yes. It uses the IANA time zone database through your browser, so each city follows its own rules for the date you enter. If a government changes its rules, the update arrives with your browser or operating system. Time zone converter
How is age in years, months and days calculated?
Full years are counted first, then full calendar months, then the remaining days. That is the convention used by most official forms. It is not the same as dividing the total number of days by 365 or 30, which is why two calculators can disagree by a day. Age calculator
What about people born on 29 February?
In non-leap years the age calculator treats 28 February as the last day before the birthday, so the year ticks over on 1 March. Some jurisdictions use 28 February instead; check the rule that applies to your form if the exact day matters.
Is anything I type stored or sent?
The calculations run in your browser, there is no account, and this site has no database of what you type. Two things to know: the countdown timer writes the target date into the page address so it can be bookmarked, while the event name and the birthday go into a link only when you press Copy share link; and the time zone converter keeps your list of cities in this browser. The site also runs a session recording service (Microsoft Clarity): the tool area of every page, input and results, is masked from it. The details are in the privacy policy. Privacy policy