About the Add Days Calculator
This calculator adds or subtracts a whole number of days from a start date using standard Gregorian calendar arithmetic. Enter a start date and the number of days to move — use a negative number to subtract — and choose whether to count every calendar day or only weekdays (Monday through Friday).
How the calculation works
- Calendar days: the result is simply the start date plus N days on the calendar, rolling over month and year boundaries automatically. February 20 plus 10 days is March 2 (or March 1 in a leap year, since February then has 29 days).
- Weekdays only: the calculator steps forward (or backward) one day at a time and only counts Monday through Friday, skipping every Saturday and Sunday. Adding 5 weekdays to a Friday lands on the following Friday, not the following Wednesday, because two weekend days are skipped along the way.
- Leap years: handled automatically — February has 29 days in any year divisible by 4 (except century years not divisible by 400), so date math around February always accounts for this.
Reading the results
- Resulting date is the start date shifted by the entered number of days.
- Day of week shows what weekday the result falls on.
- Days apart is the exact number of calendar days spanned between the start date and the result — in weekday mode this can be larger than the number you entered, since skipped weekends still add to the calendar span.
- ISO week is the ISO 8601 week number (1–53) of the resulting date, where week 1 is the week containing the year's first Thursday.
Common uses
- Project deadlines and milestone planning
- Legal and contractual date calculations (90-day cure periods, notice windows)
- Delivery and shipping estimates counted in business days
- Personal planning: countdown dates, anniversaries, and due dates
Precision note
This calculator treats Saturday and Sunday as the weekend and does not exclude public holidays. If your deadline depends on a specific holiday calendar or a jurisdiction's own counting rule (some count the start day, some don't), verify the result against that specific rule.
Frequently Asked Questions
How do I add or subtract days from a date?
Enter a start date and a number of days, then press Calculate. Use a positive number to move forward (add) or a negative number to move backward (subtract). The calculator applies standard Gregorian calendar arithmetic, so it automatically handles month-end rollovers and leap years.
What's the difference between calendar days and weekdays?
Calendar days count every day, including Saturdays and Sundays. Weekdays-only mode skips weekends entirely, counting only Monday through Friday. Adding 5 weekdays to a Friday lands on the next Friday, while adding 5 calendar days lands on the following Wednesday.
Does this calculator account for leap years?
Yes. It uses standard Gregorian calendar arithmetic, which includes a February 29 in leap years (years divisible by 4, except century years not divisible by 400). Month and year rollovers are handled automatically.
Does weekdays-only mode exclude public holidays?
No. It only skips Saturdays and Sundays. Public holidays vary by country, region, and industry, so this calculator does not build in any specific holiday calendar — subtract holiday dates manually if your calculation needs to exclude them.