Date Duration Calculator Guide: Days, Weeks, Months Explained A date duration calculator answers three different questions. ’How much time has passed’ needs a past date - days since you started, since you met, since launch. ’How much time is left’ needs a future date - days until the exam, the wedding, the deadline. ’What date falls N days out’ needs a start date plus a number - 90 days from today, 30 days after invoice, 14 days notice. The third is the one people get wrong by hand, because it steps across months of unequal length. Count in layers: full weeks first, then leftover days, then hours. Full guide: visit https://avenzum.com