Should I include the end date?
It depends on what you're counting. From Monday to Friday is 4 days apart, but a Monday-to-Friday trip covers 5 days. Leave 'include the end date' off to measure the gap between dates (like days until an event), and turn it on to count every day in a span (like days of a vacation or a hotel booking period).
Weekdays and business days
The weekday count includes every Monday to Friday in the range. It doesn't skip public holidays, because those differ by country, state and employer. To get business days for a US deadline, subtract any federal holidays that fall in the range, such as Thanksgiving or the Fourth of July.
Handy uses
- Countdowns: days until a wedding, holiday, due date or vacation.
- Deadlines: the date 30, 60 or 90 days after a notice, invoice or purchase, using add / subtract mode.
- Work: how many weekdays a project or notice period covers.
- Anniversaries: how long you've been together, or at a job, in years, months and days.
- Leap years are handled automatically, including 29 February.
How the days are counted
The total is the number of midnights between the two dates. From 1 January to 2 January is 1 day; from 1 January to 31 December in a normal year is 364, or 365 with the end date included. Weeks are the total divided by seven, with the leftover days shown separately.
The years, months and days breakdown counts whole months first, then the remaining days, the same way the age calculator does. Daylight saving changes don't affect the result, because only calendar dates are compared.
30, 60 and 90 days
Notice periods, return windows, payment terms and visa stays are often written in days. 90 days is not the same as 3 months. From 1 January they happen to match, both landing on 1 April in a normal year. From 1 June, 90 days lands on 30 August but 3 months lands on 1 September. Use add / subtract mode with the exact number of days the rule gives.