Date calculator
Date arithmetic comes in three shapes: moving forwards or backwards from a date, measuring the gap between two, and looking up facts about one. All three are here, because the question people actually have is usually one of them and they rarely know which name it goes by.
How to use the date calculator
The one rule worth carrying away is that days and months behave differently. A day count is a fixed number of days; a month keeps the day-of-month number and clamps when the target month is shorter. That difference is invisible until a month-end date is involved, at which point it becomes the whole answer. Everything else in date arithmetic is bookkeeping.
Questions
Adding to and subtracting from dates, gaps between two dates, and facts about a single date.
Yes; the date fields default to today, filled in by your browser.
Yes, and the difference matters at month ends. A month keeps the day number and clamps; 30 days does not.
A week numbering where week 1 contains the first Thursday of the year, so early January can fall in the previous year week 52 or 53.
Throughout; including 29 February as a start or end date.