Date Calculator (add/subtract days, or difference between dates)
Two modes: 'add or subtract' to compute a future or past date, or 'between' to find the difference in years, months, days, weeks, and business days.
How it works
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.
Frequently asked questions
›What happens if I add 1 month to Jan 31?
You get Feb 28 (or Feb 29 in a leap year). Calendar arithmetic clamps to the last day of the target month when the source day doesn't exist.
›Are holidays subtracted from business days?
No — only weekends. Subtract the country-specific holidays manually based on your jurisdiction's calendar.
›Does it handle negative date arithmetic?
Yes via the 'subtract' mode. Internally, subtraction is just adding negative deltas. The 'difference between' mode always returns positive values regardless of order.
›What time zone does this use?
Your browser's local time zone. Dates are interpreted as midnight local. Cross-time-zone calculations may need adjustment.
›Can I add hours and minutes?
Not yet — date granularity only. Use a separate timestamp tool for hour-level math.
›What's the largest difference it handles?
Hundreds of years comfortably. JavaScript's Date supports ±100 million days from 1970, so this is the practical limit.
›Why does it count both endpoints differently?
Total days is the count of nights between the two dates. Two consecutive days have a difference of 1 day. Business day count walks through each calendar day in between.
›Does the data leave my browser?
No. All math runs locally.
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