How days between dates are counted
By default the calculator counts the number of days from the start date up to, but not including, the end date. This is how most deadlines, notice periods and interest calculations work: from 1 January to 2 January is one day. Tick Include the end date when both the first and last days count, as with a holiday booking or a hotel stay described in days.
Worked example: from Thursday 1 January 2026 to Friday 25 December 2026 is 358 days, or 51 weeks and 1 day. Of those, 256 are weekdays (Monday to Friday).
Adding months and years
Months have different lengths, so adding a month means "the same day next month". If that day does not exist, the calculator uses the last day of the month: 31 January plus one month is 28 February (29 February in a leap year). Banks and lenders use the same rule for monthly payment dates.
Leap years
A year is a leap year if it divides by 4, except century years, which must divide by 400. So 2028 is a leap year, 2100 will not be, and 2000 was. The calculator handles this automatically.
Public holidays are not excluded. Holidays differ by country and region (for example, US federal holidays, UK bank holidays that differ between England and Scotland, Canadian statutory holidays by province, and Dutch national holidays such as Koningsdag). Subtract them yourself for legal or payroll deadlines.
Frequently asked questions
How do I count the days between two dates?
Subtract the earlier date from the later one. The calculator counts from the start date up to the end date; tick "Include the end date" if both days should count.
How many business days are between two dates?
The calculator counts weekdays from Monday to Friday. It does not remove public holidays, which vary by country.
What date is 90 days from today?
Enter today's date, choose Add, enter 90 and choose Days. The result shows the date and the day of the week.
What happens when I add a month to 31 January?
February has no 31st, so the result is the last day of February: the 28th, or the 29th in a leap year.