Counting days on a calendar is error-prone, especially across months and years. This date calculator does two jobs: it counts the days, weeks and weekdays between two dates, and it adds or subtracts a number of days from a date to find a deadline, due date or anniversary. It follows the real calendar, so month lengths and leap years are handled automatically.
How to use the date calculator
- Choose what you want to do: count days between dates, or add/subtract days.
- Enter the start date.
- For a count, enter the end date and decide whether the end date itself should be counted.
- To add or subtract, enter the number of days — use a negative number to go back in time.
Date formula and counting rules
Weeks = ⌊ days ÷ 7 ⌋ weeks and (days mod 7) days
New date = start date + N days
By default the start day isn't counted, matching how most people and most deadlines count: from Monday to Friday is 4 days. Weekdays are the Mondays through Fridays in the counted span; holidays are not removed.
Worked examples
- January 1 to December 31, 2026: 364 days (365 including December 31), or 52 weeks exactly. That span contains 260 weekdays; 261 if December 31 is included.
- October 15 to December 25, 2026: 71 days, of which 51 are weekdays.
- 90 days after October 15, 2026: Wednesday, January 13, 2027.
- 30 days before October 15, 2026: Tuesday, September 15, 2026.
Days in each month
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in leap years) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
A year is a leap year if it's divisible by 4, except century years, which must be divisible by 400 — so 2000 was a leap year and 2100 won't be.
Where date math comes up
- Deadlines: “within 30 days” or “90 days from purchase” return windows and notice periods.
- Countdowns: days until a trip, wedding or holiday.
- Durations: the length of a project, lease or pregnancy in weeks and days.
Inclusive vs. exclusive counting
Whether to count the end date depends on the question. “How many days until my trip?” doesn't count the travel day itself, so leave the end date out. “How many days does the conference last?” counts every day from the first to the last, so include it — a conference from March 3 to March 5 is three days, while March 5 minus March 3 is only two. Contracts and legal deadlines may have their own counting rules, such as excluding the day of notice or extending deadlines that fall on weekends; check the document itself.
Need hours rather than days? Use the Hours Calculator.
Frequently asked questions
How do I count days between two dates?
Subtract the earlier date from the later one. Most date calculations, including this one by default, don't count the start day — from January 1 to January 2 is 1 day. Choose “include it” if you need to count both the first and last day, such as for the length of an event.
How many weekdays are in a year?
Usually 260 or 261 Monday-to-Friday days, depending on which day the year starts. 2026 has 261 weekdays. Public holidays are not subtracted.
What is 90 days from today?
Choose “Add or subtract days”, enter today's date and 90. For example, 90 days after October 15, 2026 is Wednesday, January 13, 2027.
Does the calculator handle leap years?
Yes. It uses the calendar directly, so February 29 is counted in leap years such as 2028.
Are holidays counted as business days?
The weekday count includes every Monday through Friday. Holidays vary by country, state and employer, so subtract any that apply to you.