How it’s calculated
Calendar days and weeks: the start date is moved forward (or back) by exactly that many days (a week is 7 days), crossing month and year ends and leap days automatically. Business days: the calculator steps one day at a time and counts only Monday to Friday, so weekends are skipped. The start date itself is not counted. Months and years keep the same day of the month; if that day does not exist in the target month, the last day of the month is used (31 January + 1 month = 28 February, or 29 February in a leap year; 29 February 2024 + 1 year = 28 February 2025).
Example: Friday 9 October 2026 + 30 calendar days = Sunday, 8 November 2026. Friday 9 October 2026 + 5 business days skips the weekend: Mon 12, Tue 13, Wed 14, Thu 15, Fri 16, so the result is Friday, 16 October 2026.
Public holidays differ by country and employer and are not removed; add a day for each holiday in the period.
Frequently asked questions
Is the start date counted?
No. Adding 1 day to Monday gives Tuesday. This matches how most deadlines ("within 30 days") are counted, but check the rules that apply to you.
What if the start date is a weekend and I add business days?
The first business day counted is the next Monday (or the previous Friday when subtracting).
What is 31 January plus one month?
February has no 31st, so the result is the last day of February: 28 February, or 29 February in a leap year.
Are holidays skipped?
No, only Saturdays and Sundays. Add one day for each public holiday in the period.
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Sources
- Introduction to Calendars — U.S. Naval Observatory
- Julian Date Converter — U.S. Naval Observatory
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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