Calendar days vs business days
This tool counts calendar days: every date on the calendar, including Saturdays, Sundays and public holidays. That is the right convention for things like visa windows, medication schedules, return periods and "valid for 90 days" stamps. Many deadlines, though, are written in working days, where weekends do not count and 10 days can mean two full weeks. If that is what your deadline needs, use the business days calculator instead, and read the wording of your rule carefully, because the two conventions can land on dates a week or more apart.
You can add days or weeks here, but not months, and that is deliberate. A week is always exactly 7 days, so "6 weeks from now" has one correct answer. A month has no fixed length: adding "one month" to 31 January could mean 28 February, 1 March, 2 March or 3 March depending on the rule you pick, and different software picks differently. When a period is genuinely defined in months, anchor it to the same day number in the target month and decide explicitly what happens when that day does not exist.
Frequently asked questions
How do I add business days instead of calendar days?
Use the DaySums business days calculator, which skips Saturdays and Sundays. This page counts every calendar day, so adding 10 days to a Monday lands on a Thursday, while adding 10 business days lands two full weeks later on a Monday. Deadlines in contracts and court rules often specify which kind of day they mean, so check the wording before choosing.
Is the start date counted?
No. The start date is day zero, so adding 1 day to Monday gives Tuesday. This matches the everyday meaning of phrases like "in 30 days" and is the convention used by most deadline rules. If your rule counts the start date as day one, subtract one from the amount before calculating.
What is the largest span supported?
36,500 days, roughly 100 years. Practical uses of day arithmetic, such as contract deadlines, visa windows, warranty periods and anniversaries, fall well inside a century. Beyond that range the answers stop being meaningful for planning, and very large numbers are usually a typo, so the calculator flags them instead of silently producing a far-off date.