Date Difference Calculator

• Calendar days (inclusive both ends) — Mar 1 to Mar 5 = 5 days (counting both Mar 1 and Mar 5).
• Calendar days (exclusive of start, inclusive of end) — same range = 4 days. The "until" interpretation: 4 days "until" Mar 5 starting Mar 1.
• Calendar days (exclusive both ends) — same range = 3 days. Days "in between" not counting either endpoint. Less common in everyday usage.
• Business days — same range, Mar 1 (Wed) to Mar 5 (Sun) = 3 working days (Wed, Thu, Fri).
• Business days excluding holidays — if Mar 3 is a holiday in your jurisdiction, drop to 2 days.

Most legal and contractual contexts use one specific convention. "Net 30 days" means the invoice payment is due 30 calendar days after issue date (not 30 business days). "30 working days" or "30 business days" excludes weekends and statutory holidays. Misreading the convention is the #1 cause of payment-deadline disputes.

• Easter-linked holidays move every year — Good Friday, Easter Monday, Whit Monday, Corpus Christi. Algorithmic computation possible but rarely worth implementing yourself; use a calendar library (workalendar, holidays for Python; date-fns for JS does not include holidays — use date-holidays).
• Substitute days — if a public holiday falls on a weekend, some countries (UK, Japan) shift the observed holiday to the next Monday. Some (Germany, US federal) do not.
• Half-day holidays — Christmas Eve, New Year's Eve in many European countries. Banks close at noon; offices may or may not. Not consistent.
• Local holidays — Bavarian holidays differ from Berlin's; Catalonia from Madrid; Quebec from Ontario. National holiday APIs typically miss these.
• New holidays — Juneteenth became a US federal holiday in 2021. Holiday databases need updating; many do not get the most recent additions.

"One month from January 31" — when? February 31 does not exist. Most libraries pick February 28 (or 29 in leap years), some pick March 3, some throw an error. Adding consecutive single-month operations is not associative: (Jan 31 + 1 month) + 1 month = Feb 28 + 1 month = Mar 28, but Jan 31 + 2 months = Mar 31. Be explicit about your convention.

For invoice due dates, the right convention is usually "end of month if origin is end of month, else same day next month". For age calculations, the right convention is "same day next month, anchor to last day if shorter". Different APIs default to different conventions.

• You are checking a contract deadline that says "20 working days" and want to know the exact end date including the relevant country's holidays.
• You are planning a project schedule and need realistic delivery dates that respect weekends and statutory holidays.
• You are computing eligibility periods (warranty expiration, return policy windows, lease renewal deadlines) where the exact day matters.
• You are comparing dates from different jurisdictions and need both perspectives — "30 days from now" in business terms differs by country.

• It will not know about your company's private holidays. The calendar includes statutory/public holidays for each country, not company-specific PTO policies, religious observances not legally recognized in that jurisdiction, or local-only holidays below the federal level.
• It will not handle time-zone-dependent date arithmetic. "1 January 2026 in Tokyo" and "1 January 2026 in New York" are 14 hours apart in real time. The calculator treats dates as calendar abstractions, not specific moments in time.
• It will not predict future holiday calendars. Some holidays (Easter) are algorithmic and can be projected indefinitely. Others (one-off royal jubilees, government-declared bank holidays for events) are added by the holiday-data maintainer; future-only-declared holidays may not be in the database yet.