Date Calculator — Step-by-Step Solutions

The Date Calculator helps you work with dates in four modes: find the number of days between two dates, add or subtract days from a date, count business days (excluding weekends), and determine what day of the week a date falls on.

Enter your dates and get instant results. The calculator handles leap years, month boundaries, and provides results in days, weeks, months, and years.

Start Date

End Date

Your calculations will appear here

How to Use the Date Calculator

Days Between Dates: choose a start and an end date. You get the total days, weeks plus days and the exact difference in years, months and days. The order of the two dates does not matter.
Add/Subtract Days: choose a start date, enter the number of days, and choose whether to add or subtract. The result date and its day of the week are shown.
Business Days: choose two dates to count weekdays and weekend days between them. Counting starts the day after the first date and includes the second date. Public holidays are not excluded.
Day of the Week: choose any date to see its weekday, whether it is a weekend, its day of the year and its ISO week number.

Formula and Theory

Dates are counted on the calendar, where months have 28 to 31 days and leap years add 29 February. Because of that, you cannot reliably work with dates by assuming 30-day months. Differences are best counted as whole days, and converted to weeks by dividing by 7.

A leap year is a year divisible by 4, except for century years, which must be divisible by 400. So 2000 and 2024 are leap years but 1900 and 2100 are not. This is why 365 days after 29 February 2024 is 28 February 2025.

Business days are the days from Monday to Friday. The count here treats each calendar day after the start date up to and including the end date and removes Saturdays and Sundays. For project planning with public holidays, subtract those separately.

The ISO week number follows the ISO 8601 standard: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. Because of this rule, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next.

Worked Examples

Example 1: Days until an event

Problem: How many days are there from 1 March 2025 to 25 December 2025?

Solution: From 1 March to 1 December is the sum of the month lengths in between: 31 (March) + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 (November) = 275 days. Then 24 more days reach 25 December: 275 + 24 = 299. Weeks: 299 / 7 = 42 remainder 5.

Answer: 299 days = 42 weeks and 5 days (9 months and 24 days)

Example 2: Adding days to a date

Problem: What date is 90 days after 15 January 2025?

Solution: 15 January plus 16 days is 31 January. Add 28 days for February (2025 is not a leap year): 44. Add 31 days for March: 75. The remaining 90 - 75 = 15 days reach 15 April.

Answer: 15 April 2025, a Tuesday

Example 3: Business days in a project window

Problem: How many working days are there from Monday 2 June 2025 to Friday 13 June 2025?

Solution: There are 11 calendar days from 2 June to 13 June. Counting starts the day after the first date, so the days counted are 3 to 13 June. The weekend falls on 7 and 8 June, which is 2 days. Business days = 11 - 2 = 9: Tuesday 3 to Friday 6 (4 days) plus Monday 9 to Friday 13 (5 days).

Answer: 9 business days (2 weekend days)

Example 4: Leap day arithmetic

Problem: What date is 365 days after 29 February 2024?

Solution: From 29 February 2024 to 28 February 2025 is 365 days, since the next 29 February does not exist in 2025. One day later, 1 March 2025, is 366 days after.

Answer: 28 February 2025

Frequently Asked Questions

How many days are between two dates?

Subtract the earlier date from the later one. The tool counts whole calendar days, so the difference between 1 March and 2 March is 1 day.

How do I add 30, 60 or 90 days to a date?

Use the Add/Subtract Days mode. Enter the start date and the number of days and the tool returns the new date and its day of the week.

Does the business day count include holidays?

No. It only excludes Saturdays and Sundays. If your calendar has public holidays in the period, subtract them from the result.

Is the start date counted?

In the total days and business days, the start date is not counted and the end date is. If you need an inclusive count of both ends, add 1.

How is the week number calculated?

It is the ISO 8601 week number: weeks run Monday to Sunday, and week 1 contains the first Thursday of the year. This matches most European business calendars; some countries number weeks differently.

Does it account for leap years?

Yes. The calculation uses real calendar dates, so 29 February is included when it falls in the range.