Date Math

Date & Time Calculator

Do the date math calendars make hard: shift a date by years, months, weeks or days, count the gap between two dates, and turn a birthday into an exact age with a next-birthday countdown. Month-end and leap-year edge cases are handled for you, and nothing leaves your browser.

Add / subtract Business days Age in Y/M/D Runs locally

Modes: Add · Difference · Age
Edge cases: Leap year · Month end
Privacy: 100% local
Dependencies: None

Base date

Years Months Weeks Days

Adding months keeps the day-of-month when possible. If the target month is shorter, the date is clamped to its last day (Jan 31 + 1 month → Feb 28/29).

Start date End date

Calendar days count the elapsed days between the two dates. Business days exclude Saturdays and Sundays.

Birth date

Age is counted in whole years, months and days from the birthday. Feb 29 birthdays are treated as Feb 28 in non-leap years.

This session

Records live only in this page’s memory and disappear when you reload or close the tab.

Built for everyday date math

Three calculators in one page

Everything you normally count on your fingers or in a spreadsheet, done instantly and consistently.

Shift any date

Add or subtract years, months, weeks and days in a single pass — deadlines, renewals, reminders and delivery windows.

Measure the gap

See the exact distance between two dates in days, weeks and a years / months / days breakdown.

Business days

Count Mon–Fri working days between two dates for project schedules, SLAs and leave planning.

Exact age

Turn a birthday into years, months and days, with the date and day count of the next birthday.

Edge cases handled

Leap years, month-end clamping and weekends are calculated in UTC date arithmetic, so no result drifts by a day.

Nothing to install

No sign-up, no upload, no ads in the way. Every calculation happens locally in your browser.

How date arithmetic works

The small rules that make date calculations agree with a paper calendar.

What does “plus one month” actually mean?

A month is not a fixed number of days, so date calculators anchor on the day-of-month instead: Jan 15 + 1 month is Feb 15. When the target month is shorter, the day is clamped to the last valid day — Jan 31 + 1 month is Feb 28 (or Feb 29 in a leap year), and Mar 31 − 1 month is Feb 28/29.

Why do days, weeks and months give different totals?

“Total days” is a fixed count of 24-hour periods between the two calendar dates. “Weeks + days” groups that count by 7. The years / months / days breakdown walks the calendar instead, so it matches how people say an interval out loud (1 year 2 months 3 days) rather than a simple division.

What counts as a business day?

Business days here are Monday through Friday. Public holidays are not included, because they differ by country and region — subtract them manually if your schedule needs it.

Why use UTC for the calculation?

Dates without times have no time zone. Calculating in UTC avoids daylight-saving transitions, when a local day can be 23 or 25 hours long and simple division would round to the wrong day.

Common deadline cheatsheet

The date spans people actually count at work and in paperwork, with the rule that decides the exact day.

Scenario	Typical length	How it is counted
Payment terms 30 / 60 / 90 days	30, 60 or 90 calendar days	From the invoice or delivery date; confirm whether the trigger day itself counts.
Probation or notice period	1–6 months	Counted in months, not fixed days, so month ends clamp (Jan 31 + 3 months = Apr 30).
Visa or visa-free stay	90 calendar days	Usually from the day after entry; the departure day often does not count.
Pregnancy	40 weeks / 280 days	From the first day of the last menstrual period to the estimated due date.
Return or warranty window	7, 14 or 30 days	Check whether the terms say calendar days or business days before subtracting.
Financial settlement T+1 / T+2	1–2 business days	Skips weekends and local public holidays, so the calendar date moves.
Contract “within 90 days”	90 calendar days	State whether the start day is included — this is where disputes come from.

When a contract only says “days”, assume calendar days, and write the start date explicitly.

Date formats that avoid ambiguity

The same eight digits can mean two different days. These are the formats worth recognising.

Format	Example	Where you see it
ISO 8601	2026-10-26	APIs, databases, filenames, international standards
RFC 3339 (with time)	2026-10-26T14:30:00+08:00	Web APIs and logs that include a time zone
Day / Month / Year	26/10/2026	UK, most of Europe, Australia, Latin America
Month / Day / Year	10/26/2026	United States, Philippines — the usual source of mix-ups
Chinese	2026年10月26日	Chinese contracts, invoices and official documents
Written English	26 October 2026	Contracts and certificates; the month spelled out removes doubt

For anything cross-border, prefer ISO 8601 or spell the month. All-numeric DD/MM versus MM/DD is the riskiest choice.

Frequently asked questions

Is the start or end date included in the day count?

The tool reports elapsed days: from Jan 1 to Jan 2 is 1 day, and from Jan 1 to Jan 1 is 0 days. If you need an inclusive count (for hotel nights, for example), add 1 to the result.

Does it handle leap years and February 29?

Yes. Feb 29 is a valid input, and adding one year to Feb 29 lands on Feb 28 in the next non-leap year. For age, Feb 29 birthdays are treated as Feb 28 in non-leap years.

Can I calculate dates far in the past or future?

Any date the calendar supports from year 1 to 9999 works, including historical dates and long-range planning. Results are exact for the proleptic Gregorian calendar.

Can I use it for workday planning?

Yes for Monday–Friday counting. Because holidays vary by country, the tool intentionally does not guess them; subtract your own holiday list from the business-day result.

Is my data sent anywhere?

No. The page is static plus client-side JavaScript: dates never leave your browser, and there is no account or server-side storage.

Age, anniversaries and February 29

Different contexts count age differently — pick the convention that fits the question.

Full years (age)

You are N years old from your birthday until the day before the next one. Born 2000-05-15, you turn 18 on 2018-05-15, not on New Year’s Day.

Nominal age (East Asia)

The traditional 虚岁 / 세는나이 counts a person as 1 at birth and adds a year at New Year, so it can be one or two years above the legal age.

Legal thresholds

Most jurisdictions reach a legal age at the start of the birthday; a few use the exact birth time. For contracts and travel documents, follow local rules.

Anniversaries and Feb 29

Anniversaries advance by years or months with month-end clamping. For a Feb 29 birthday, non-leap years commonly use Feb 28, but some legal systems use Mar 1.

A short history of the calendar

Why a year is not exactly 365 days, and why the date jumped ten days in 1582.

45 BC

The Julian calendar

Julius Caesar replaces the lunar civic calendar with a solar one and adds a leap day every four years. It overshoots the tropical year by about 11 minutes.
325

Council of Nicaea

The church fixes the date of Easter, which eventually turns the calendar’s slow drift into a religious and civil problem.
1582

The Gregorian reform

Pope Gregory XIII drops ten days — Thursday 4 October is followed by Friday 15 October — and adds the 100/400-year leap rule. Britain and its colonies switch in 1752.
1970

The Unix epoch

1970-01-01 00:00 UTC becomes the zero point for Unix time. It is an instant, not a calendar date, which is why timestamps need a time zone.
1988

ISO 8601

The standard fixes YYYY-MM-DD and the Monday-based week numbering still used in payroll, logistics and project planning.

The calendar in daily use today is the Gregorian calendar; the leap-year rule you notice every four years came from that 1582 fix.

Related tools

Need time instead of dates? These tools cover the other half.

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Compare the same instant across cities and time zones, including daylight-saving offsets.

Countdown Timer

Count down to a deadline, launch or birthday with a live timer.