The story behind the date calculator
Counting days sounds simple until a month has 28 days, a year has a leap day or Easter moves by a month. The rules behind those quirks are centuries old, and the date calculator follows them exactly.
The Gregorian reform
The Julian calendar of 46 BC added a leap day every four years, which made the year about 11 minutes too long. By the sixteenth century the calendar had drifted ten days away from the seasons. Pope Gregory XIII fixed this in 1582: Thursday 4 October was followed directly by Friday 15 October. Many countries switched much later, Great Britain only in 1752.
The leap year rule
Since the reform a year is a leap year when it is divisible by 4, except century years, which must also be divisible by 400. So 2000 was a leap year but 1900 was not. This gives an average year of 365.2425 days, very close to the solar year. The calculator uses this rule for every date, which is why 29 February only appears when it should.
ISO 8601 week numbers
The international standard ISO 8601, first published in 1988, defines weeks that start on Monday. Week 1 is the week that contains the first Thursday of the year, or equally the week with 4 January. A year therefore has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year.
Easter and movable holidays
Easter falls on the first Sunday after the ecclesiastical full moon on or after 21 March, a rule traced back to the Council of Nicaea in 325. Carl Friedrich Gauss published a formula for it in 1800, and an anonymous algorithm printed in Nature in 1876 is still widely used. Holidays such as Good Friday, Ascension Day and Whit Monday are counted from Easter, so they move every year too.
Frequently asked questions
Why does adding one month to 31 January give 28 February?
February has no 31st day. The calculator then takes the last day of the target month, which is how most banks and contracts handle month ends.
Which countries have built in public holidays?
The Netherlands, Belgium, Germany, France, the United Kingdom (England and Wales) and the United States. Only national holidays are included, not regional ones.
Can I use the results for legal or payroll deadlines?
The calculator follows the calendar rules exactly, but deadlines can depend on local law or extra days off. Check the official rules for anything important.
Related tools
Date calculator
A free date calculator: count the days between two dates, add or subtract months and days, count working days with public holidays, find an exact age and look up the ISO week number.
Pomodoro timer
A free Pomodoro timer: 25 minutes of focus, 5-minute breaks and a long break after four sessions, with a progress ring, sound signals, notifications and daily stats.
Meeting cost calculator
A free meeting cost calculator: add roles with hourly rates or salaries, start the live ticker and see what the meeting costs per minute. Log past meetings and copy a summary.
Eisenhower matrix
A free online Eisenhower matrix: sort tasks into Do first, Schedule, Delegate and Eliminate, drag them between quadrants, add due dates and notes. Saved in your browser.
To-do list
A free, minimalist to-do list: add tasks with Enter, tick them off, filter by status, tag and priority, sort by due date and keep several lists, all stored in your browser.
Ready to try it?
Open PictorX in your browser — no install, no credit card required to start.
Open the date calculator