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Date Calculator — Days Between & Add/Subtract

Find the exact number of days, weeks, months, or years between two dates, or add and subtract spans of time from a starting date. Includes a business-day mode that excludes weekends. Runs entirely in your browser.

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How to use

  1. Step 1: Choose mode: Days Between or Add/Subtract
  2. Step 2: Pick the start date
  3. Step 3: Enter the end date or the amount to add/subtract
  4. Step 4: Toggle 'Exclude weekends' for business-day counts
  5. Step 5: Read the result and breakdown

How date math works

The calculator uses real calendar arithmetic via JavaScript's Date object and the date-fns library, which together handle leap years, varying month lengths (28-31 days), daylight-saving-time shifts, and time-zone differences.

Days Between mode computes the absolute difference between two calendar dates. It returns the result in years (full), months (full), weeks (full), and days (total) — letting you choose whichever unit matters most.

Add/Subtract mode takes a start date and shifts it by a given amount. Adding "1 month" preserves the day-of-month when possible; if the target month is shorter (e.g. adding 1 month to Jan 31 lands on Feb 28/29), date-fns clamps to the last valid day.

Business-day mode excludes Saturdays and Sundays. It does not currently account for public holidays, which vary by country. For tighter business-day math, subtract national-holiday counts manually.

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Examples

Example 1 — Days between two dates
StartJan 1, 2026
EndDec 31, 2026
Calendar days364
Business days260
Years / Months / Weeks0 / 11 / 52
Example 2 — Add 90 days
StartMay 6, 2026
Add+ 90 days
ResultAug 4, 2026
Day of weekTuesday
Example 3 — Subtract 6 months
StartMar 31, 2026
Subtract− 6 months
ResultSep 30, 2025
NoteEnd-of-month clamping handled automatically

Frequently asked questions

A date calculator is a tool that performs arithmetic on calendar dates. It can find the exact span between two dates or add and subtract days, weeks, months, or years from a starting date — accounting for leap years and varying month lengths.

Why use a date calculator?

  • Project deadlines — count business days from kickoff to delivery
  • Contract terms — find the exact end date of a lease, NDA, or warranty
  • Visa & immigration — verify "X days within last Y" eligibility windows
  • Shipping & logistics — estimate ETA in business days from order date
  • Birthdays & anniversaries — count down precisely
  • Loan and bill due dates — find the next due date after rolling forward

Business days, holidays & time zones

Business days are typically defined as Monday-Friday, excluding weekends. This calculator follows that convention. Public holidays are not subtracted because they vary by country, region, and even employer (banking holidays vs federal holidays vs school calendars). For the United States in 2026 there are roughly 10-11 federal holidays; subtract them manually if your use case demands it.

Time zones are handled by your device's local clock. Both dates use the same zone, so the result is always consistent — but if you're computing the gap between events scheduled in different zones, do the conversion before entering them here.

Common mistakes & tips

  • Off-by-one errors. "Days between Mon and Tue" is 1 day, not 2 — this calculator returns the count of full days elapsed.
  • Ambiguous month math. Adding "1 month" to Jan 31 lands on Feb 28 (or 29 in leap years), not on Mar 3.
  • Confusing business days with bank days. Banks observe additional federal holidays — always check.
  • Year boundaries. Don't forget that adding 365 days lands on a different calendar date in a leap year (366 days).