Days between two dates, add or subtract days, day of the week and business-day calculator.
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Pick "days between two dates", "add or subtract days from a date", or "business days only". The mode determines which input fields appear and how the calculator interprets weekends and holidays.
Type or pick the start date. Use a four-digit year so historical dates (1920s, 1940s) and future dates (2030s) are unambiguous and immune to the 2-digit year roll-over problem.
For "days between" mode, pick the end date. For "add/subtract" mode, type a positive number of days to advance the date or a negative number to go back. The calculator handles month-length and leap-year math automatically.
For business-day mode, the calculator subtracts weekends. Where a holiday list is supported, public holidays in your jurisdiction are also subtracted — useful for legal deadlines, T+2 settlement and SLAs.
The result shows the gap or new date plus breakdowns in weeks, months and years. Watch the inclusive vs exclusive convention — this calculator returns exclusive count by default, matching most legal systems.
Date arithmetic operates on calendar dates — a tuple of (year, month, day) on the Gregorian calendar. The two fundamental operations are: (a) date — date = duration in days, and (b) date + duration = new date. Internally, calculators convert dates to a "Julian day number" or Unix-style day count (days since a fixed epoch like 1 January 1970), do integer arithmetic, then convert back. This avoids all the edge cases of month lengths and leap years because the conversion functions handle them once. For business-day calculations, the calculator also subtracts weekends and (optionally) public holidays from the count.
Conversion to a Julian day number is a closed-form formula and is bijective with the proleptic Gregorian calendar. Once both dates are integers, the difference is trivial. For business days, count whole 7-day weeks (each contributes 5 business days), then add the leftover days excluding any weekends and public holidays that fall in the leftover range.
2024 is a leap year, so the answer is 366 days from 1 January to 31 December inclusive — or 365 days exclusive of one endpoint.
Most date calculators (including this one) return 365 (exclusive of the start date): 31 December is 365 days after 1 January in 2024.
15 February 2025 + 90 days = 16 May 2025.
Walk-through: Feb has 13 days remaining (after the 15th), March 31, April 30 ≈ 13+31+30 = 74 days bring us to 30 April. Then 16 more days into May = 16 May.
The calculator does this in one operation by converting to JD, adding 90, and converting back.
Total days: 39 (inclusive of both endpoints — adjust to your preferred convention). Weekends: 6 Saturdays + 6 Sundays = 12 weekend days. Public holidays (US): 25 December (Christmas), 1 January (New Year's Day) = 2 holidays (both on weekdays).
Business days = 39 → 12 → 2 = 25.
This is exactly the kind of calculation lawyers, project managers and finance teams need for deadlines and settlement dates.
Convert both to Julian day numbers (or Unix epoch days) and subtract. The exclusive count gives "days until" (the number of midnights crossed); the inclusive count adds 1. This calculator returns the exclusive count by default, matching how most legal and scheduling systems work.
Subtract weekends (Saturdays and Sundays) and public holidays from the total day count. For long ranges you can approximate: business days — total × 5/7. For accurate counts under a few weeks, list the weekends and holidays explicitly.
A leap year has 366 days (Feb 29 added). The rule: divisible by 4 is a leap year, EXCEPT century years (divisible by 100) which are NOT, EXCEPT those also divisible by 400 which ARE. So 2000 was a leap year; 1900 was not; 2024 is; 2100 will not be.
There are two conventions. (1) Roll forward: 31 January + 1 month = 3 March (skips Feb 28/29). (2) Clip: 31 January + 1 month = 28 February (or 29 in a leap year). Most spreadsheet software and legal contracts use clipping. This calculator uses clipping by default.
It uses the proleptic Gregorian calendar with year 0 = 1 BC. So "1 January 1 BC" is year 0; "1 January 100 BC" is year −99. Historical dates pre-1582 are not the dates that contemporaries would have written — they were on the Julian calendar.
Divide the day count by 7. For partial weeks: 23 days = 3 weeks and 2 days. For pregnancy or training programs that use full weeks, take the integer part of the division.
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