Date Calculator: days between, business days, ISO weeks

How dates are counted

Days between counts from the start date up to the end date, so a Monday to the next Monday is 7 days. Including the end date adds one day.
Adding months or years keeps the day of the month. When the new month is too short, the result is its last day: January 31 plus one month is February 28, or February 29 in a leap year. The extra days can roll over into the next month instead.
Years and months come first, then weeks and days. Years, months and days between two dates are counted the same way, so adding the answer to the start date gives the end date.
Adding business days starts from the day after the start date. With a Saturday and Sunday weekend, one business day after a Friday is the next Monday.
Quarters are calendar quarters: January to March is Q1.
Sunday-start week numbers begin each week on Sunday and put January 1 in week 1.

Leap years are the years divisible by 4, except century years, which must be divisible by 400: 2000 was a leap year and 1900 was not.
ISO 8601 weeks start on Monday. Week 1 is the week that contains the year's first Thursday, which is also the week that contains January 4. A year has 52 or 53 weeks, and up to three days at either end of a year can belong to a week of the neighboring year: Sunday, January 3, 2021 is in week 53 of 2020.
Dates follow the Gregorian calendar for every year from 1 to 9999, including the years before it came into use.