Date and age calculator
Two dates. Put a birthday in the first box and it is an age calculator; put any two dates in and it is the gap between them, counted every way at once.
This one runs on your own device. It is worked out in your browser, so there is nothing to send and nothing to log. Where a tool on this site does send something out, that page says so and names where it goes.
A month is not a fixed length, and this says which one it used
Every calculator that tells you an age in years, months and days has made a decision it usually does not mention. There is no such thing as “a month” as a quantity: February is 28 days, March is 31, and which one you are counting depends on where in the year you are standing.
The count here works forwards from the earlier date. It advances a month at a time for as long as it can without passing the later date, and whatever is left over is the days. So the months are real calendar months — the ones you actually lived through — rather than an average of 30.44 days, which is the other common choice and which gives a different answer.
Why 31 January plus a month is 28 February
There is no 31st of February, so adding a month to the 31st of January has to land somewhere, and the convention is the last day of the month it lands in. That is a sensible rule with an odd consequence: it does not run backwards. Add a month to 31 January 2024 and you get 29 February; take a month off 29 February and you get 29 January, not the 31st you started from.
This is the date equivalent of adding and then removing the same percentage and not getting back to where you began. The reading above says so whenever your start date is late enough in the month for it to matter.
Seven days or eight?
From the 1st to the 8th is seven days if you are counting the gap and eight if you are counting the days involved. Both are right and they answer different questions. A hotel charges you for seven nights; a contract that runs from the 1st to the 8th inclusive covers eight days; a notice period usually excludes the day it was given.
The default here is the gap, because that is what “how long between these dates” means. The option above switches it, and the reading says which one it used every time rather than leaving you to guess.
Where to go next
Dates and durations, from the other direction:
- hours worked calculator — the same arithmetic inside a single day
- running pace calculator — time per unit rather than time between dates
What this does not do
Everything is counted in whole days, with no time of day and no time zones. That is deliberate: the moment a clock is involved, daylight saving makes some days 23 hours long and others 25, and a calculator that silently returns 7.958 days has helped nobody.
Dates before 1752 in Britain are not what you think they are. The country changed calendar that September and eleven days were removed from it, so a gap spanning that date is a gap between two different calendars. This counts straight through as though the change never happened, which is the usual convention and is wrong if you are doing genealogy.