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Two date jobs, done precisely: the difference between any two dates (in days, weeks, and approximate months and years), and date arithmetic — what's 90 days from today, what was 6 weeks before a deadline. Month and year arithmetic uses real calendar rules, so adding a month to January 31 lands sensibly and leap years are respected.
Frequently asked questions
Does "days between" include both endpoints?
It counts the gap between the dates — Monday to Friday is 4 days. If you need inclusive counting (common for billing or medication), add one to the result.
What happens when I add a month to January 31?
JavaScript-style calendar arithmetic rolls overflow forward: January 31 + 1 month lands on March 2 or 3 (depending on leap year), because February lacks a 31st. For end-of-month bookkeeping, work from the 1st and subtract a day.
Why are months and years shown as approximate in the difference?
Because months vary from 28 to 31 days, a difference in months isn't a fixed quantity. The day and week counts are exact; months use the 30.44-day average and are labelled as estimates.