AgeGapCalc

AgeGapCalc answers one question: how far apart are two birthdays? Enter a birth date for each person, with optional names, and the result shows who is older and by how much, in years, months and days, plus a single running total of days. A swap button fixes dates entered the wrong way round, and a copy button grabs the result for a message. Nothing is uploaded; the calculation happens in your browser.

The page makes a fair point about why this is worth a dedicated tool. Subtracting two dates in a spreadsheet and dividing by 365 gives a decimal like 3.27 years, which is not how people describe an age gap and is slightly wrong because of leap years. Subtracting the parts by hand can produce nonsense such as one year, minus two months and five days. AgeGapCalc uses calendar borrowing instead, taking a month's worth of days when the day value goes negative and a year when the month value does, and it clamps month lengths so 31 January and 29 February are handled properly.

The site lists the obvious uses: couples comparing birthdays, parents working out how many school years separate their children, and coaches checking which age bracket a player falls into. It is free with no sign-up and says it collects no data. The same site has an age calculator for one person, a date duration tool, a half-birthday finder and a blog of related explainers.