What this calculator does
This tool takes a start date and a number of days, then returns the resulting calendar date. You can add days to project a future date or subtract days to find a past date. An optional business-days mode counts only weekdays, skipping Saturday and Sunday so that weekends never consume part of your day total. It works for any year using the proleptic Gregorian calendar, so leap years are handled automatically.
How to use it
Pick your Date Format (mm/dd/yyyy, dd/mm/yyyy or ISO yyyy-mm-dd) — this only controls how dates are entered and shown, never the math. Enter the start month, day and year, choose Add or Subtract, and type the number of days. Tick "Business Days Only" to count weekdays only. If your workplace treats Saturday as a working day, also tick "Saturday is also a Business Day" so that only Sunday is skipped.
The formula explained
For plain calendar days the start date is converted to a serial day number, the signed day count is added, and the serial number is converted back to a Gregorian date:
$$\text{resultDate} = \text{startDate} + (\text{sign} \times \text{days})$$For business-day mode the calculator walks one day at a time: each step moves the date forward (or backward) by a single calendar day, but the remaining counter only drops when that day is a business day:
$$\text{remaining} = \text{remaining} - 1 \quad \text{if day is a business day}$$This guarantees the result always lands on a working day. The start date itself is never counted; counting begins on the next or previous day.
Worked example
Start on Monday June 15, 2026, add 30 calendar days. June has 30 days, so 15 days reach June 30 and 15 more reach July 15. The result is Wednesday July 15, 2026 — because June 15 is a Monday and 30 days is four weeks plus two days, landing two weekdays later.
Key Terms Defined
- Calendar day
- Any single day of the week, including weekends and holidays. Adding calendar days simply advances the date one unit at a time with no skipping.
- Business day
- A day on which normal business is conducted — conventionally Monday through Friday. In this calculator, weekends are skipped when "business days only" is selected; public holidays are not automatically excluded.
- Weekend
- The non-working days of the week. By default this means Saturday and Sunday. With the "Saturday is a business day" option enabled, only Sunday is treated as the weekend.
- Serial day number
- An integer that represents a date as a count of days from a fixed reference point (an epoch). Date arithmetic is performed by converting each date to its serial number, adding or subtracting the day count, then converting back — which automatically handles varying month lengths and leap years.
- Proleptic Gregorian calendar
- The Gregorian calendar extended backward to dates before its official 1582 introduction. Using it lets a calculator apply one consistent set of leap-year rules across all historical dates without switching to the older Julian calendar.
- Leap year
- A year with 366 days (February 29 added). In the Gregorian system a year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400. So 2000 and 2024 are leap years; 1900 and 2100 are not.
- Start-date exclusion convention
- The rule of whether the start date itself counts as day zero or day one. When adding days, the start date is typically excluded (day 1 is the day after), so "add 1 day" to Monday yields Tuesday. Different tools may include the first day, which shifts results by one — always confirm which convention applies.
FAQ
Does it count the start date? No. The first counted day is the day after (for add) or before (for subtract) the start date.
Are public holidays excluded? No. Only weekends can be skipped in business-day mode. To allow for holidays, add extra days manually.
Does it handle leap years? Yes. Subtracting 1 day from March 1, 2024 correctly returns February 29, 2024.
