Days between two dates: how to count them right
Count correctly, with examples you can reproduce in the tool: the off-by-one trap, months of unequal length and the cases that change the result.
There are 30 days from January 1 to January 31, not 31, unless you also count the starting day. That off-by-one is the most common mistake when counting days between dates, so it helps to know what each calculation counts. The date calculator gives the difference between two dates in years, months and days, plus totals in days, weeks, months and hours, in your browser with nothing sent anywhere.
Does the first day count?
It depends on what you need to know. There are two ways to count:
| Counting method | Jan 1 to Jan 31 | Typically used for |
|---|---|---|
| Difference (start day not counted) | 30 days | Deadlines, how long until, how long since |
| Inclusive (both ends counted) | 31 days | Hotel nights versus days, work shifts |
The date-difference mode subtracts one date from the other, so January 1 to January 31 gives 30. The business-days mode, by contrast, counts both ends. If a lease, a notice or a contract says "within 30 days", check whether day zero is the day of the notice or the day after: the wording decides, not the calculator.
Years, months and days
The tool breaks the gap down the way a person reads it. From October 6, 2026 to December 25, 2026 is 2 months and 19 days, 80 days in total. Calendar months differ in length, so "months" is never "days divided by 30": complete calendar months are counted and the leftover days follow.
Leap years
February has 29 days in leap years, which are the years divisible by 4, except century years that aren't divisible by 400. That changes real answers: February 28 to March 1 is 2 days in 2024 and 1 day in 2023. The tool handles it on its own, and it's a good case for checking any hand calculation. Month ends follow the same rule as adding: from January 31 to March 1, 2026 you get 1 month and 1 day, because January 31 plus one month is February 28 and March 1 is one day later.
Adding and subtracting dates
To find the date 90 days from now, use the Add or subtract mode. Starting from October 6, 2026, 90 days lands on January 4, 2027. With months and years it doesn't spill into the next month: January 31 plus one month is February 28, 2026, not March 3, and February 29, 2024 plus one year is February 28, 2025.
Exact age
The age mode calculates completed years, months and days and the days left until the next birthday. Someone born May 15, 1990 is, on October 6, 2026, 36 years, 4 months and 21 days old, with 221 days to go until they turn 37.
Business days
The business-days mode counts Monday to Friday between two dates, both included, and lets you paste a list of holidays, one date per line. From October 5 to October 16, 2026, with no holidays, that's 10 business days. For work deadlines and filings, see how to count business days (with US holidays).
Weeks and hours
The weeks total is the number of complete weeks: 80 days is 11 weeks, because the tool drops the 3 leftover days. Total hours are simply days times 24, so they don't reflect a clock change: they are calendar hours, not wall-clock hours. If you need exact hours between two moments that include a time of day, see the timestamp converter.
Typical counting mistakes
- Adding an extra day by counting inclusively when the rule was a difference.
- Dividing days by 30 to get months, which differs from calendar months.
- Forgetting February 29 when crossing a leap year.
- Counting a deadline from the date on a document when the rule counts from the day it was delivered.
Time zones don't matter
All calculations use calendar dates, not instants. So daylight saving or your browser's time zone can't shift a result by a day: two people, one in Mexico City and one in Madrid, get the same answer for the same two dates. If you need to convert a timestamp that includes a time of day, use the timestamp converter.
Using the calculator, step by step
- Open the date calculator and pick a mode: difference, add or subtract, business days or age.
- Choose the dates. If the start date is after the end date, the tool warns you.
- Read the result and, if you need to compare it with another, note it down before switching modes.
Frequently asked questions
How many days are there between two dates?
Subtract one date from the other. From January 1 to January 31 is 30 days if you don't count the starting day, and 31 if you count both ends. It depends on whether your rule is a difference or inclusive.
How do I add days to a date?
Choose the add or subtract mode, enter the starting date, the amount and the unit. From October 6, 2026, 90 days lands on January 4, 2027.
Why does the result change in a leap year?
Because February has an extra day. February 28 to March 1 is 2 days in 2024 and 1 day in 2023.
How many months are there between two dates?
The tool counts complete calendar months plus leftover days, rather than dividing days by 30.
How do I calculate my exact age?
Use the age mode: enter your birth date and you get completed years, months and days, plus the days until your next birthday.
Are my dates sent to a server?
No. Everything is calculated in your browser.
Calculate dates without mistakes
Date difference, add or subtract, business days and exact age. Runs locally, free, no signup.
Open Date Calculator →Related tools
- Date Calculator: difference, add or subtract, business days and exact age.
- Business Days: count working days with Mexico, Spain or US holidays.
- Timestamp Converter: convert timestamps that include a time of day.
- Unit Converter: convert between units of measure.
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