Date Calculator (add/subtract days, or difference between dates)

What it computes

Add/subtract mode: takes a date, adds or subtracts a number of years, months, and days, and gives you the resulting calendar date. Years and months use calendar arithmetic (so adding 1 month to Jan 31 gives Feb 28 or Feb 29 depending on leap year, not Mar 3). Days are exact — adding 30 days always advances exactly 30 calendar days.

Between mode: takes two dates and gives the difference in years/months/days plus totals in days, weeks, months, and business days (Monday through Friday, no holidays). The Y/M/D form uses the same borrow logic as our age calculator — a difference is e.g. '2 years, 5 months, 3 days', not a decimal year.

Why business days don't include holidays

We count Monday through Friday as business days and exclude Saturday and Sunday. Public holidays vary by country, region, and year, so we don't subtract them automatically. For US business-day work, subtract roughly 10 federal holidays per year from the result; for UK, about 8 bank holidays; for Japan, about 16.

If holidays matter for your calculation (deadline tracking, contract negotiations), do the calendar count here, then check your country's holiday list manually. We may add country-specific holiday subtraction in a future version.

Common use cases

Project deadlines: 'this is due 90 days after kickoff'. Add 90 days to the start date.

Contract terms: count days between contract start and end. Match against your contract language for billing.

Travel planning: how many business days between leaving and returning? Useful for out-of-office calendars and expense reports.

Personal milestones: anniversaries, sobriety counts, time since a major life event.