Add days to a date

How adding days to a date works

For calendar days the calculator simply moves forward (or back) one day at a time, counting every day including weekends, and crossing month and year boundaries with the correct month lengths and leap years. The start date is day zero, so the first day counted is the day after it. For business days it moves day by day as well, but only Monday to Friday increase the count; Saturdays and Sundays are stepped over.

Worked example: start on Thursday 15 January 2026 and add 30 calendar days. January has 31 days, so 16 days take you to 31 January and 14 more land on Saturday 14 February 2026. Because that is a weekend, a contract that needs a working day would usually roll forward to Monday 16 February. Adding 10 business days to the same Thursday gives Thursday 29 January: two full working weeks, which span 14 calendar days.

Getting deadlines right

Before you trust a result, check which counting rule actually applies. Many notice periods, payment terms such as net 30, and return windows are written in calendar days, while court filings, bank transfers and shipping promises are often stated in business days. The two can differ by weeks: 90 business days is roughly 126 calendar days.

Watch the starting point too. Some rules say the first day counted is the day after an event (the convention used here); others count the event day as day one, which moves the answer one day earlier. Public holidays are not removed automatically because they vary by country and region, so if a holiday falls inside a business-day range, add one extra day for each. Finally, when a calendar-day deadline lands on a weekend, local rules decide whether it moves to the following Monday or stays put; the warning above tells you when that question applies.