Date & Age Finder
Three calculators in one. Measure the days between two dates, look up the week number of any date, or work out an exact age in years, months and days — all with your choice of date format.
What this tool does
The Date & Age Finder bundles the three time calculations people look up most, each on its own tab. Date Difference measures the gap between any two dates. Week tells you the ISO week number of a date and lets you add or subtract time to land on a new date. Age turns a birth date into an exact age and a set of life-in-numbers. Pick a tab, choose your dates with the calendar picker, and the results update instantly.
Choosing your date format
Different countries write dates in different orders, and that ambiguity is a common source of mistakes — 03/04/2026 is the 3rd of April in London and the 4th of March in New York. The Format switch at the top right controls how every date is displayed across all three tabs, either DD/MM/YYYY (day first, used across most of the world) or MM/DD/YYYY (month first, used in the United States). The calendar picker itself is unambiguous, so you can enter dates confidently and simply read them back in whichever order you prefer.
Days between, and days including both
"How many days between two dates" has two honest answers, and this tool shows both. Days between counts the gap — the nights, if you like — so the 1st to the 2nd is one day. Days including both dates counts every calendar day the range touches, both endpoints included, so the 1st to the 2nd is two days. The first is what you want for "how long until"; the second is what you want for "how many days am I booking the hotel" or "how many days does this festival run". Keeping them side by side removes the off-by-one error that catches almost everyone.
Calendar span versus total days
A duration can be expressed two ways. The total days count is a single running number. The calendar span breaks the same duration into years, months and days — which is how humans actually talk about age and anniversaries. Because months have different lengths, the breakdown uses a clamped-anchor method: it advances whole months from the start date first, clamping the day where a month is too short (the 31st of January plus one month lands on the last day of February, not a spillover into March), then counts the leftover days. This is why the same gap can read as "30 days" and "1 month" depending on the months involved, and both are correct.
How week numbers work
The Week tab uses the ISO 8601 standard, the one businesses and calendars rely on. Under ISO rules a week starts on Monday, and week 1 is the week containing the year's first Thursday — equivalently, the week containing January 4th. A consequence worth knowing: the last days of December can belong to week 1 of the next year, and the first days of January can belong to week 52 or 53 of the previous year. The tool reports the correct ISO week and the year that week belongs to, so a date like December 31st may show as week 1 of the following year by design, not by error.
Leap years and the edges
Every calculation accounts for leap years — the every-four-years rule, minus century years, plus the every-four-hundred exception that keeps 2000 a leap year and 1900 not. Dates are handled in a fixed time reference so that daylight-saving changes never add or drop an hour from a day count. Where a calculation is genuinely undefined — a "to" date earlier than the "from" date, or a birth date in the future — the tool says so rather than returning a misleading negative number, and for the date difference it will simply measure the gap regardless of which way round you enter the two dates.
Privacy
Every date you enter stays in your browser. Nothing is uploaded, logged, or sent anywhere — the entire tool runs offline on your device.