Days and months are different units
A day is always a day, and a week is always 7 of them. A month is not a fixed length: it can be 28, 29, 30 or 31 days, so “in 3 months” and “in 90 days” land on different dates almost every time. From March 15, three months later is June 15, but 90 days later is June 13. From January 31, three months later is April 30, but 90 days later is May 1.
Which unit is right depends on who set the deadline. Statutes and contracts that say “30 days” mean days, and the business-days tab exists because many of them actually say “30 business days”. Subscriptions, rent and billing cycles usually mean calendar months, renewing on the same day number each month.
The month-end rule, and why January 31 + 1 month is February 28
Adding a month keeps the day number and changes the month. That works until the day doesn’t exist in the target month: there is no February 31. The calculator then uses the last day of that month, so January 31 + 1 month is February 28 (29 in a leap year), and March 31 − 1 month is also February 28. This is the convention used by banks for monthly payment dates, by payroll systems and by most date libraries. If you compare this result with another system, it will almost certainly agree.
One effect worth knowing: month arithmetic is not reversible at month end. January 31 + 1 month = February 28, but February 28 − 1 month = January 28. If a plan schedules something “on the 31st every month”, check the short months by hand.
Order of operations when you mix units
With years, months, weeks and days all filled in, the calculator adds the calendar part first (years × 12 + months, as one step) and then the fixed part (weeks × 7 + days). That is how people talk about it: “a year and two weeks from now” means next year’s same date, then two weeks on. Adding the months in one step also avoids drift. Twelve one-month steps from January 31 would get stuck on the 28th after February, but one 12-month step returns to January 31.
Business days: deadlines that skip weekends and holidays
“Within 10 business days” appears in refund policies, notice periods, bank transfers and government processing times. Counting starts the day after the start date, and only Monday to Friday count. Choose a holiday calendar and US federal holidays (at their observed dates) or England & Wales bank holidays (with substitute days) are skipped too. Ten business days from the Wednesday before Thanksgiving ends a day later than it would in an ordinary week. To count how many working days fall between two dates, or see a month-by-month table for the year, use the business days calculator.
Day of the year and ISO week numbers
Each result also shows its day of the year (1 to 365, or 366 in a leap year) and its ISO-8601 week number. Under ISO rules weeks start on Monday, and week 1 is the week containing the year’s first Thursday. So the first days of January can belong to the previous year’s week 52 or 53, and the last days of December can belong to the next year’s week 1. January 1, 2027 is a Friday and falls in week 53 of 2026. Many European workplaces and supply chains plan by week number, so it is worth checking the year that goes with it. To measure the gap between two dates instead, use the days between dates calculator.