How dates are added and subtracted
Adding days or weeks is exact arithmetic: 30 days after March 1 is March 31, 6 weeks is 42 days, and the tool reports both the resulting date and its weekday. Adding months or years keeps the day of the month where possible: March 15 plus 2 months is May 15. When the target month is shorter, the result is clamped to the last day of that month, so January 31 plus 1 month is February 28 (or 29 in a leap year), and February 29, 2024 plus 1 year is February 28, 2025. The tool tells you whenever this end-of-month rule kicks in. This is the same behavior as Excel's EDATE function and most programming date libraries.
Subtracting works the same way in reverse. "What date was 90 days ago" is a common question for return windows, probation periods and medical follow-ups; set the direction to Subtract and the amount to 90.
Business days
Check the business-days box and the calculator counts only Mondays through Fridays when the unit is Days. Ten business days from a Friday is two calendar weeks later, on the following-following Friday, and the result line shows how many weekend days were skipped. The start date itself is not counted, so 1 business day after Friday is Monday. Federal holidays are not excluded; if Thanksgiving or Christmas falls inside the range, add a day per holiday. Legal and contractual "business day" definitions vary, so check the governing document when it matters.
The box has no effect on weeks, months or years, because those units are calendar periods rather than counts of working days.
Everyday uses
Payment terms: net 30 from an invoice date. Return policies: 30 or 90 days from purchase. Notice periods: two weeks from today. Leases and contracts: a one-year term starting on a given date ends the day before the anniversary, so add 1 year and then subtract 1 day. Pregnancy: the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period. Medication and food expiration: production date plus the shelf life. Immigration and travel: a 90-day visa-free stay counted from entry. In every case, knowing the weekday of the result tells you whether a deadline lands on a weekend, when many rules push it to the next business day.