Date calculator

What this calculator does

Four everyday date questions in one place, each on its own tab:

• Days between two dates, also shown in weeks and in years, months and days. From 13 October to 31 December 2026 is 79 days, or 80 if both dates are counted.
• Add or subtract days, weeks, months or years. 100 days after 13 October 2026 is Thursday 21 January 2027.
• Working days between two dates, with Saturday and Sunday off (or Sunday only, or Friday and Saturday) and an optional list of holidays you type yourself. September 2026 has 22 weekdays; list two days off for the National Day holiday, 1 and 2 September, and 20 remain.
• Week and weekday of any date: the day of the week, the ISO 8601 week number and week-year, the day of the year and the quarter.

The two dates are shared between the tabs, so you can type them once and switch between counting calendar days and working days.

How to use it

1. Choose a tab. The start date is today and the end date is the last day of this year until you change them.
2. Type or pick the dates. In "Add or subtract", choose the operation, the amount and the unit.
3. For working days, choose the weekly days off and, if you like, paste holidays one per line as 2026-09-02. A name after the date is ignored, so 2026-09-02 National Day works. Lines that are not a valid date are listed by number and left out.
4. Copy the result with the copy button.

The rules behind the numbers

Counting both ends or not. By default the start date is not counted, the way a distance is measured: from Monday to Wednesday is 2 days. Tick "Include the end date" to count both, the way a hotel or a leave request often counts: Monday, Tuesday and Wednesday are 3 days. Working days always count both ends, like the NETWORKDAYS function in spreadsheets, and the count is negative when the end comes before the start.

Adding months. When the day does not exist in the target month, the result is the last day of that month: 31 January plus one month is 28 February (29 in a leap year), and the calculator says so. Adding a year to 29 February gives 28 February of the next year. This is the rule used by spreadsheets, most programming languages and banks. Adding 30 days is a different question from adding one month, and the two give different dates.

ISO weeks. Weeks start on Monday and week 1 is the week that contains the year's first Thursday. That is why 3 January 2021 belongs to week 53 of 2020 and 30 December 2024 to week 1 of 2025. Some years, such as 2026, have 53 weeks. The ISO week date (for example 2026-W42-2 for Tuesday 13 October 2026) is what many payroll, logistics and planning systems use.

Calendar. All dates use the Gregorian calendar, extended backwards (proleptic) for dates before it was adopted. The arithmetic works on calendar dates, not clock times, so time zones and daylight saving never move a result by a day.

Tips and common mistakes

• Off by one. Most disagreements about "how many days" are about whether both ends are counted. Decide first, then tick the box to match.
• Public holidays move. Lunar New Year (Tết) and other lunar holidays change date every year, and governments sometimes swap working days. Paste the official list for the year you are counting.
• Weekends differ by country. Many offices in Vietnam work on Saturday mornings; parts of the Middle East rest on Friday and Saturday. Pick the pattern that matches the contract.
• To calculate someone's exact age, use the age calculator; to compare times across countries, the time zone converter. The counting rules are worked through in how to count days and working days between dates.