What this calculates
Pick a start date and add or take away any mix of years, months, weeks and days — or a number of business days, Monday to Friday — and this gives the date you land on, its day of the week, how many calendar days away it is, and how many of those fall on a weekend.
The formula
Calendar
result = start ± years and months, then ± (weeks × 7 + days)Business days
step one day at a time, counting only Monday to FridayYears and months move first, as calendar months: if the start day does not exist in the month you land in, the date is pulled back to that month's last day. The start date itself is never counted, in either mode.
Worked example
In business days, 10 business days from Thursday 1 January 2026 is Thursday 15 January — fourteen calendar days, because two weekends (four days) are skipped.
When to use it, and the mistakes to avoid
Use it for deadlines and notice periods, delivery and payment dates, due dates counted in weeks, visa and warranty expiry dates, and anything written as "within N working days".
The mistakes that cost the most:
- Counting the start date as day one. 30 days from 1 January is 31 January, not 30 January.
- Treating a month as 30 days. One month from 31 January is 28 February, and three months from 1 January is 1 April — 90 days, not 91.
- Using calendar days for a working-days deadline. "10 working days" from a Thursday is two full weeks later, not ten days.
- Forgetting public holidays. They are not skipped here; add a day for each one inside the span.
- Forgetting leap years. 29 February 2028 plus one year is 28 February 2029.
FAQ
What date is 90 days from 1 January 2026?
Wednesday, 1 April 2026. January has 30 days left after the 1st, February 28 and March 31, which makes 89 - so the 90th day is 1 April.
Is the start date counted?
No. Adding one day to a date gives the next day, so the start date is day zero. That matches how deadlines are usually written: '30 days from 1 January' is 31 January.
What happens when I add a month to 31 January?
You land on the last day of February - 28 February 2026, or 29 February in a leap year. A month is a calendar month, so the date keeps its day number where it can and is pulled back to the month's last day where it cannot.
What are business days?
Monday to Friday. In business-day mode the calculator steps over Saturdays and Sundays: ten business days from Thursday 1 January 2026 is Thursday 15 January, fourteen calendar days later. One business day after a Friday is the Monday.
Are public holidays skipped?
No. Holidays differ by country, by state and from year to year, so only weekends are skipped. If a holiday falls inside the span, move the result on by one business day for each.
How do I count backwards from a date?
Choose Subtract. 100 days before 1 January 2026 is Tuesday, 23 September 2025. It works for business days too: one business day before a Monday is the Friday before.