Z

Date & Time

Five tools for calendar and clock arithmetic: exact age, the distance between two dates, shifting a date by days, weeks, months or years, time-zone conversion for a specific date, and a stopwatch with countdown timer. The awkward cases — month lengths, leap years, daylight saving — are where they earn their keep.

Date Difference Calculator

Find the number of days, weeks, months and years between two dates.

Private · in-browser

Date Add / Subtract

Add or subtract days, weeks, months or years from a date.

Private · in-browser

Age Calculator

Calculate exact age in years, months and days from a birth date.

Private · in-browser

Time Zone Converter & World Clock

Convert a time between time zones and see a world clock.

Private · in-browser

Stopwatch & Timer

A simple, accurate stopwatch and countdown timer that runs in your browser.

Private · in-browser

Which tool do I need?

  • Age calculator — exact age in years, months and days by calendar subtraction with borrowing. It takes a reference date, so it also answers how old someone was at a past event or will be at a future eligibility cut-off.
  • Date difference — the span between two dates as days, weeks, months and years at once — notice periods, project spans, countdowns.
  • Date add and subtract — shift a date forward or back by days, weeks, months or years. Deadlines quoted as "within 90 days" land here.
  • Time zone converter — one wall-clock time translated into other zones for a date you specify, plus a world clock.
  • Stopwatch and timer — for timing something in front of you rather than computing a date.

Two neighbors live in the developer hub: the epoch converter for Unix timestamps and the cron parser for reading a schedule in plain English.

Why date tools disagree with each other

Months have no fixed length, so "one month after 31 January" has no single correct answer. This site lets the day-of-month roll forward instead of clamping: 2025-01-31 plus one month gives 2025-03-03, because February 2025 has 28 days and the extra three spill into March. Tools that clamp return 28 February. Neither is wrong, but knowing which convention applies explains most disagreements.

Counting works the same way. Date difference reports whole units from a day count, treating a month as 30.4375 days and a year as 365.25, whereas the age calculator borrows the real length of each calendar month — so "months between" and "age in months" can differ by a day or two around month ends. Every count here is calendar days, so a 45-day contractual deadline is not 45 working days.

Time zones and daylight saving

Offsets are computed for the date you enter from the IANA time-zone database your browser ships, not a fixed table — which matters, because zones switch on different dates. New York is four hours behind London between 8 and 29 March 2026, and the usual five either side of that gap. Half-hour zones such as Kolkata at UTC+5:30 resolve exactly, large offsets push the same instant onto the next calendar day in Sydney, and clock changes create an hour that is skipped in spring and repeated in autumn. Avoid scheduling anything critical there.

Frequently asked questions

Why does adding one month to 31 January give 3 March?
Because the tool lets the day-of-month overflow roll forward rather than clamping to the end of the month. 31 January plus one month targets a nonexistent 31 February, and the surplus days spill into March — 2 March in a leap year, 3 March otherwise. Tools that clamp report 28 or 29 February instead.
Why does the date difference month count not match the age calculator?
They use different methods on purpose. Date difference converts a day count into whole units using average month and year lengths, which is what you want for a span. The age calculator does true calendar subtraction with borrowing, which is what you want for a birthday. Around month ends the two can differ by a day or two.
Does the time zone converter handle daylight saving?
Yes, and per date rather than per zone. It reads the IANA time-zone database through your browser, so a conversion for 15 March 2026 applies the DST rules in force on that day. This is why the New York to London gap is four hours for part of March and five hours the rest of the year.
Do any of these tools count business days?
No — everything counts calendar days, including weekends and public holidays, since holiday calendars differ by country and even by state. Use the date tools to land on the calendar date, then check it against your own working calendar if the deadline is defined in business days.
Where is the Unix timestamp converter?
Under Text and Developer Tools, as the epoch converter, alongside the cron expression parser. They sit there because they are about machine-readable time formats rather than everyday calendar arithmetic, but they behave the same way: pure conversion, computed in the page.