A free date calculator: how long until, how long since, and what date is 90 days from now.
Count the days between two dates, or add and subtract days, weeks and months. Working days skip weekends and, if you choose, public holidays, so a notice period or delivery deadline comes out right.
Months have 28 to 31 days, so "add a month" has no single answer. Spreadsheets, programming languages and contracts each pick a rule. We use the first one, the most common, but it's worth knowing the others exist when two systems disagree by a few days.
Counting the gap between dates gives four. Counting the days you actually turn up gives five. Neither is wrong; they answer different questions. Click days in the week to see both counts change.
Each one counts differently. Click to load it into the calculator from today.
England’s bank holidays come from rules in the Banking and Financial Dealings Act 1971 plus royal proclamations: fixed dates with weekend substitutes, “first Monday” and “last Monday” rules, and two days tied to Easter. Easter itself is the first Sunday after the ecclesiastical full moon on or after 21 March, computed by an algorithm called the computus. Pick a year to generate them all.
Zeller’s congruence (1882) and Conway’s Doomsday rule (1973) both find the weekday with pencil arithmetic; the browser’s Date object counts milliseconds from 1970. All three run on the start date from the calculator. Below them: the ISO week, the Julian Day Number astronomers use, and the same day on the old Julian calendar.
Loans, salaries and swaps set a day of the month, then a business day convention, defined in the ISDA 2006 Definitions, says where it goes when that day is a weekend or holiday. “Modified” conventions refuse to cross into another month. Here is a 12-month schedule from your chosen date, using England’s bank holidays generated above.
Mean year lengths are computed from each leap rule and compared with the mean tropical year of about 365.24219 days.
Neighbouring calculators, with a typical result for each.