This date calculator does the two jobs people actually need: move a date forward or back by a given amount, and count the distance between two dates. Business days skip weekends, month arithmetic handles short months, and the days-between result shows both the exclusive and inclusive counts because contracts disagree about which is meant.
Adding months is not adding days
Thirty days from 31 January is 2 March; one month from 31 January is 28 or 29 February. Both are defensible, and contracts mean different things by them — which is why days between two dates and the 30/60/90-day conventions are worth reading before relying on either.
Inclusive or exclusive counting
From Monday to Wednesday is two days if you count the gaps and three if you count the days themselves. The calculator shows both, so you can match whichever your deadline uses. Business-day counting skips weekends but not public holidays, which vary by country — see counting business days.