Three date calculation modes
Dates accompany almost every project: professional deadlines, administrative due dates, birthdays, contract durations or simple curiosity about elapsed time. This calculator offers three modes to answer the most common needs without opening a paper calendar or counting days on your fingers.
The first mode calculates the duration between two dates. You enter a start date and an end date, and the tool gives the total number of days, then breaks it down into years, months and days. This is ideal for checking the seniority of a contract, the length of a seasonal rental, or the number of days between two events. The second mode adds or subtracts a number of days from a start date: handy for knowing the due date of a 30-day deadline, or for going back 90 days before a deadline. The third mode calculates age or seniority from a single date: it details the years, months and days up to today.
How to read and use each result
The duration calculation distinguishes calendar days from working days. Calendar days count every day without exception, including weekends and public holidays. Working days exclude Saturdays, Sundays and French public holidays from the count: this is the value most often used in project management, employment law or for setting a legal cooling-off period.
Adding and subtracting days is simple: a start date, a number of days, and the tool gives the arrival date. If you choose to count only working days, the calculator automatically skips weekends and public holidays to land on the next available working day. This is useful for estimating a delivery date, an administrative response date or the end of a trial period.
The age calculation provides detail in complete years, months and days, not just a subtraction of years. So between 15 March 1990 and 10 March 2026, the tool will show 35 years, 11 months and 23 days, not 36 years. This precision avoids classic errors in seniority or legal age calculations.
Concrete examples
For a summer rental from 15 July to 28 July, the calculator shows 13 calendar days, which often clarifies discussions about the number of nights billed. For a 14-day cooling-off period starting on 3 January, adding 14 working days lands on 21 January, taking into account the 1 January public holiday already passed, but watch out for later public holidays depending on the chosen year.
A project manager can check that a task starting on 4 February and planned for 45 working days finishes before a release fixed for 15 April. An individual can calculate their exact age on an important birthday, or check how long separates two life milestones. Each result remains indicative: for legal or tax deadlines, always verify with the texts in force and the public holidays applicable to your territory.
Frequently asked questions about the date calculator
What is the difference between calendar days and working days?
Calendar days count every day without exception. Working days exclude Saturdays, Sundays and public holidays. It is the working-day mode that is most often used for professional and administrative deadlines.
Does the tool take French public holidays into account?
Yes, the working-day mode includes fixed French public holidays (1 January, 1 May, 8 May, 14 July, 15 August, 1 November, 11 November, 25 December). Movable feasts such as Easter or Ascension are not handled automatically in this version.
How do I calculate my exact age with this tool?
Use the Age / seniority mode by entering your date of birth. The tool shows your age in complete years, months and days as of today, without artificially rounding up to the next year.
Can I calculate a due date with a day delay?
Yes. Choose the Add / subtract days mode, enter the start date and the number of days. The tool gives the corresponding due date, with or without taking weekends and public holidays into account.
Are my dates sent or stored?
No. All calculations are performed locally in your browser. No entered date is sent to a server or kept after the page is closed.