Durations, age, time zones — 100% local.
All calculations are performed entirely in your browser, using the time zones (IANA) built into your device.
Adding or subtracting dates looks simple, but months aren't the same length, leap years add a day every four years (except century years not divisible by 400), and the switch to daylight saving makes days of 23 or 25 hours. "In one month" starting from 31 January has no obvious answer. This tool counts durations using the real calendar, not a "1 month = 30 days" approximation.
The result is given in two complementary ways: in years, months and days (the human reading — "2 years, 3 months and 12 days"), and as a total number of days (the reading useful for a calculation: interest, notice period, limitation period). The two can't be derived from one another without knowing the exact dates, because of the variable length of months.
Age is the number of birthdays already passed at the reference date. Someone born on 29 February only officially "ages" in leap years; the convention used here moves their birthday to 1 March in other years.
The conversion uses the IANA database (Europe/Zurich, America/New_York…) built into your system, which knows the history of each zone's clock changes. The same gap between two cities can therefore vary by an hour depending on the time of year, because countries don't change their clocks on the same dates — or at all.
The calculation gives the gap between the two dates. From 1 to 2 January is 1 day. If your deadline "runs until" the 2nd inclusive, add a day.
From each country's official rules, embedded in your browser. No server is queried.
No. Everything is computed locally and nothing is kept after the tab is closed.