About the Add Days Calculator
"What date is 100 days from now?" is one of the most searched date questions, and for good reason. Deadlines, delivery estimates, trial periods, medication courses, probation periods, visa stays, fitness challenges and savings goals are all described in days. Counting forward on a calendar is tedious and error-prone once the count crosses a month end, and very easy to get wrong by one day.
This add days calculator adds or subtracts any number of days from a start date and gives the resulting date and weekday. It also shows the number of weeks and days, what the date would be if you counted weekdays only, and a quick ladder of common offsets — 7, 14, 30, 60 and 90 days — from the same start date.
How to Use the Add Days Calculator
Enter the start date.
Choose add days or subtract days.
Enter the number of days.
The resulting date appears immediately.
How Days Are Added
result = start date + n days
The start date is day 0; the next day is day 1.
29 September 2026 + 1 day = 30 September 2026
29 September 2026 + 100 days = 7 January 2027The calculator converts the date into a running day count, adds the days, and converts back, so month lengths and leap years are handled exactly.
Step-by-Step Example
100 days after Tuesday 29 September 2026.
Rest of September: 1 day → 30 September (1)
October: 31 days → 31 October (32)
November: 30 days → 30 November (62)
December: 31 days → 31 December (93)
January: 7 days → 7 January 2027 (100)The result is Thursday 7 January 2027. Because 100 days is 14 weeks and 2 days, the weekday moves on by two, from Tuesday to Thursday.
Weekdays Only
If the count should skip weekends — for example a "100 working days" target — the answer is very different. Counting only Monday to Friday, 100 weekdays after 29 September 2026 is Tuesday 16 February 2027, about six weeks later than 100 calendar days. Public holidays are not skipped in this figure; the deadline calculator lets you list holidays to exclude.
Common Offsets
The calculator lists the dates 7, 14, 30, 60 and 90 days from your start date. From 29 September 2026 these are 6 October, 13 October, 29 October, 28 November and 28 December. These offsets cover most practical needs: a week's notice, a fortnightly cycle, a 30-day return window, a 60-day trial and a 90-day payment term or visa stay.
Weekday Shortcut
Adding a multiple of 7 days always lands on the same weekday. So to find the weekday of any result, divide the number of days by 7 and look at the remainder: the weekday moves forward by that many days. 100 ÷ 7 leaves 2, so Tuesday becomes Thursday. 365 ÷ 7 leaves 1, which is why the same date moves forward one weekday each common year.
Subtracting Days
Counting backwards is just as useful. Subtract 30 days from a deadline to find when to start preparing; subtract the delivery time from an event to find the last day to order; subtract the length of a visa stay from its end date to find when it began. Thirty days before 29 September 2026 is Sunday 30 August 2026.
Milestones in Days
People often track personal milestones in days: 100 days of a new habit, 1,000 days of a relationship, 10,000 days old. Businesses use them too — the first 100 days of a new manager, 90 days of probation, 30-day and 60-day follow-ups. The add days calculator turns any of these into a date you can put in a calendar, and the reverse calculation can be done with the days until calculator.
Days in Health and Travel
Health instructions are often given in days. A 7-day course of antibiotics started on a Monday finishes on the following Sunday if the first dose was on Monday, because the start day is itself day one of the course — a case where counting inclusively matters. Quarantine and isolation periods, the timing of booster doses and waiting times after vaccination are also expressed in days. For travel, many visa rules allow a stay of up to 90 days within any 180-day period, so knowing the exact date 90 days from arrival is essential. In each case, check whether the rule counts the first day, and adjust by one if it does.
Understanding Your Result
The headline is the resulting date and weekday.
The ISO date line gives it as year-month-day.
The in weeks line splits the days into weeks and days and warns if the result is on a weekend.
The weekdays only line gives the date if only Monday to Friday are counted.
The common offsets line lists 7, 14, 30, 60 and 90 days from the start.
When Should You Use This Calculator?
Use it to find the date a set number of days from today.
Use it for payment terms, trials, returns and notice periods.
Use it to plan challenges and milestones counted in days.
Use it to count backwards from a deadline.
Common Mistakes
Counting the start date as day one. Adding 1 day to Monday gives Tuesday.
Confusing days with weekdays. "10 working days" is two weeks, not ten days.
Treating 30 days as a month. 30 days after 31 January is 2 March, not 28 February.
Forgetting public holidays in business-day counts. Use the deadline calculator, which can skip the holidays you list.