Add Days to Date Calculator

How the date is calculated

• Days and weeks are exact: one week is always 7 days.
• Working days skip Saturdays and Sundays. Adding 5 working days to a Monday gives the next Monday.
• Months and years move the calendar date. If the target month is shorter, the result is the last day of that month: 31 January + 1 month = 28 February (29 in a leap year).

Worked examples

• 5 October 2026 + 90 days = 3 January 2027.
• 5 October 2026 − 45 days = 21 August 2026.
• Adding 10,000 days to a date of birth gives the "10,000-day birthday", about 27 years and 4 months later.

Where this is useful

• Deadlines: payment terms such as "net 30", return windows, notice periods and visa stays.
• Health: medication courses, follow-up appointments, and the estimated due date, which is 280 days after the first day of the last period (see the due date calculator).
• Projects: working-day estimates turned into a calendar date.
• Subscriptions and trials: when a 14-day trial or a 12-month contract actually ends.

Calendar days or working days?

Contracts and laws specify which they mean, and the difference is large: 30 calendar days is about 4 weeks, but 30 working days is 6 weeks. When a deadline falls on a weekend or public holiday, many rules move it to the next working day, but not all, so check the specific terms.

Counting from day zero or day one

"Within 7 days of 1 March" usually means by 8 March, counting 2 March as day one. Some legal systems count the starting day; most do not. This calculator does not count the start date, which matches the most common interpretation.

Common mistakes

• Treating a month as 30 days: 3 months after 31 October is 31 January, but 90 days after it is 29 January.
• Forgetting public holidays when counting working days; subtract them yourself.
• Time zones: dates here are calendar dates, so time zones do not change the result.

Common offsets at a glance

Starting from 1 January 2027:

Weekday tricks

• Adding any multiple of 7 days keeps the same weekday: a Monday plus 28 days is a Monday.
• One year later (365 days) the weekday moves forward by one; after a 29 February it moves by two. That is why your birthday falls one weekday later each year.
• Most years have 261 working days (Monday to Friday); 2026 and 2027 both have 261, and 2028 has 260, before public holidays.

ISO week numbers

Many companies in Europe and Asia plan in ISO week numbers. ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday, so a year has 52 or 53 weeks. Adding whole weeks with this calculator keeps the weekday, which makes week-based schedules easy to plan.

Months versus days

"Six months from today" and "180 days from today" are usually different dates, because months have 28 to 31 days. Contracts normally specify which they mean; if they say months, use the month option, which keeps the same day of the month (or the last day, if the month is shorter).

Calendar days or business days in a deadline?

Deadlines are written both ways, and the difference is large. "Within 14 days" normally means calendar days, weekends included. "Within 10 business days" skips Saturdays and Sundays. Starting on Monday 5 October 2026, both happen to land on Monday 19 October 2026, because two full working weeks equal two calendar weeks. Start on a Wednesday instead and the two counts drift apart, and public holidays push a business-day deadline further out. The calculator counts the day after the start date as day 1, which matches how most contracts and shipping estimates are read; check the wording if a document says the start day itself counts.

Frequently asked questions

Sources

• ISO 8601: Gregorian calendar dates and week numbering.

Last reviewed 5 October 2026. Results are estimates; see our disclaimer.