Free Date Difference Calculator

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

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Enter your details on the left, then press Calculate.

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What is this calculator for?

You signed a 30-day return policy on a purchase. The receipt is dated March 18. You want to know the exact last day you can return. Or you're planning a 90-day project — what's the deadline if you start tomorrow? Or you're a parent calculating when your baby will be 6 months old. The date calculator handles "what date is N days from X" and "how many days between X and Y" — the two most common date math questions.

Date arithmetic seems easy but has gotchas. Months have different day counts (28, 29, 30, 31). Leap years complicate February (29 days in years divisible by 4, except century years not divisible by 400). Days of week shift unpredictably across leap years. "Business days" vs "calendar days" matters for some contexts (court deadlines, banking, shipping). This calculator handles all of these accurately.

For business-day calculations specifically: business days exclude weekends (Saturday, Sunday) and standard US federal holidays (New Year's, MLK Day, Presidents Day, Memorial Day, Juneteenth, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving, Christmas). Specific industries may have additional holidays (banks observe Columbus and Veterans Day; many businesses don't). Court systems have their own specific holiday lists.

How to use this calculator

For "date after N days": enter a start date and number of days. The calculator returns the target date. Specify "calendar days" (all days) or "business days" (weekdays excluding holidays).

For "days between two dates": enter start and end dates. The calculator returns the number of days, optionally as calendar days or business days.

For "add or subtract months/years": enter start date and specify months or years to add/subtract. Useful for anniversary dates, lease/contract expiration, year-anniversary milestones.

For day-of-week lookup: enter any date past, present, or future to know which day of the week it falls on. Useful for event planning, historical research, scheduling.

Understanding your results

The calculator returns the target date, the day of week, and contextual information (the date's relationship to common holidays, working-day count, etc.).

Common date math:

30 days from May 1 = May 31. 30 business days from May 1 = June 12 (assuming Memorial Day observed).

1 year from today = 365 or 366 days (leap year handling automatic). The calculator typically returns the same day of the year (next year), accounting for leap years.

90 days from January 1 = April 1.

From Pearl Harbor (Dec 7, 1941) to D-Day (June 6, 1944): 911 days. From 9/11/2001 to today: 8,952 days (as of May 14, 2026, about 24.5 years).

The business-day complication. 14 business days from Friday May 9, 2025: Friday May 30 (skipping 2 weekends + Memorial Day). 14 business days from Monday: simpler, lands 2 weeks 4 days later. Banking and legal deadlines typically count business days, not calendar days. Shipping and delivery estimates typically use business days (FedEx "Ground delivery in 5 business days" means weekdays only, plus holiday adjustments).

The "Tuesday of the third week of next month" problem. Specifying dates by relative position requires careful interpretation. "First Monday in May 2025": May 5. "Third Wednesday in May 2025": May 21. "Last Friday in May 2025": May 30. Each interpretation is unambiguous; verify which counting your context uses (some count from Sunday, some from Monday).

A worked example

Marcus signed a 30-day money-back guarantee on a $1,200 mattress on April 7, 2026. He wants to know the exact deadline to return it.

April 7 + 30 days = May 7, 2026. The 30th day after April 7 falls on Thursday May 7. He has until end of day Thursday May 7, 2026 (or possibly midnight on May 7 depending on how the merchant interprets "30 days").

If the policy specifies "30 business days": April 7 (Monday) + 30 business days. Skip weekends and Memorial Day (May 26, 2026). Counting: May 22 falls on Friday. Counting 30 business days from April 7 (Mon): about 6 weeks later = May 19, 2026 (the 30th business day). If they specify business days, deadline is later.

Variation: Lisa is having a baby. Doctor estimates due date is November 23, 2026. She wants to know when the baby will be 6 months old (for solid food introduction milestone).

November 23, 2026 + 6 months = May 23, 2027. That's the 6-month birthday. Baby will be approximately 182 days old.

Long-horizon variation: she's curious how many days have elapsed since her parents' wedding date of August 14, 1979 to today (May 14, 2026). 1979-08-14 to 2026-05-14: 46 years 9 months = 17,074 days. Translated: 2,439 weeks, or 56 lunar cycles, or about 410,000 hours of marriage. The arithmetic produces interesting framings; the calculator handles the leap-year accumulation automatically.

Related resources

For age-specific date math, see Age Calculator. For timezone-aware scheduling, the Timezone Converter. For pregnancy-specific date math, the Due Date Calculator and Pregnancy Week Calculator. The OPM Federal Holidays page publishes the official US federal holiday calendar; timeanddate.com handles international holiday and timezone questions.

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Frequently asked questions

Does the business days count include holidays?

No. Business days here are defined as Monday through Friday. Federal or state holidays are not excluded — for payroll, project planning, or court deadlines, check the relevant holiday calendar separately.

Why are the total days and the years/months/days view different?

They describe the same duration in two ways. Total days is a single integer. Years/months/days breaks that integer into calendar units, where months are variable length (28-31 days), so the conversion is not a fixed ratio.

How do I count business days vs calendar days?

Calendar days include all days; business days exclude weekends and federal holidays. Federal holidays: New Year's (Jan 1), MLK Day (3rd Monday January), Presidents Day (3rd Monday February), Memorial Day (last Monday May), Juneteenth (June 19), Independence Day (July 4), Labor Day (1st Monday September), Columbus/Indigenous Peoples Day (2nd Monday October), Veterans Day (Nov 11), Thanksgiving (4th Thursday November), Christmas (Dec 25). Banks observe all federal holidays plus some additional state holidays. Court systems have specific holiday lists. Industry-specific: stock markets observe some holidays differently (Good Friday closure, Christmas Eve and New Year's Eve early close).

How does leap year work?

Years divisible by 4 are leap years (366 days). Exception: century years (1900, 2000) are NOT leap unless also divisible by 400. So 2000 was leap, 1900 wasn't, 2100 won't be. The rule corrects calendar drift caused by Earth's actual orbit (365.2425 days). Leap day is February 29. People born on February 29 (~1 in 1,461 of population, about 200,000 living Americans) typically celebrate on Feb 28 or March 1 in non-leap years; legal age calculations vary by state. The next several leap years: 2024, 2028, 2032, 2036.

What day of the week was a specific historical date?

Calculator handles instantly. Manual calculation uses Zeller's congruence or similar algorithms. Famous date day-of-week lookups: July 4, 1776 (US Declaration) — Thursday. December 7, 1941 (Pearl Harbor) — Sunday. November 22, 1963 (JFK assassination) — Friday. July 20, 1969 (moon landing) — Sunday. September 11, 2001 — Tuesday. The calculator handles any date; useful for genealogy, history research, biography work, planning anniversary celebrations.

How many days are in a year?

365 in standard years, 366 in leap years. Average over the 400-year leap cycle: 365.2425 days. Earth's actual orbit: 365.2422 days. The 0.0003-day discrepancy accumulates very slowly — about 1 day per 3,000 years. The Gregorian calendar (current) corrected this in 1582; it's accurate enough to last another 5,000+ years. Some other calendars track different cycles: lunar (~354 days/year — Islamic calendar), lunisolar (12 or 13 months alternating — Hebrew, Chinese), solar (similar to Gregorian — most Asian and Western use). For US business and personal life, the Gregorian solar calendar is universal.

What's the difference between 'days' and 'business days' in legal contexts?

Significant — and the specific rule varies by court, statute, and contract. Generally: 'days' (in legal contracts) means calendar days unless specified otherwise. 'Business days' explicitly excludes weekends and federal holidays. Court deadlines often use 'court days' (excluding weekends, federal holidays, AND court-specific holidays). Statutes of limitations typically use calendar days. Lease termination notices: usually calendar days. Court filing deadlines: court days. Always read the specific contract or statute language; ambiguity goes to the party seeking enforcement. If a contract says '30 days,' err on the side of calendar days (shorter timeline); if it says 'business days,' the timeline is longer. The Mubboo date calculator can compute both interpretations.