Date Duration Calculator
How many days are between two dates?
Whether you are tracking a project deadline, counting down to an event, or verifying a contract period, knowing the exact number of days between two dates saves time and prevents mistakes. A lease starting on March 15 and ending on September 30 spans 199 calendar days – but how many of those are actual working days? Manual counting is slow and error-prone, especially across months and years with different lengths.
The date duration calculator below solves this instantly. Enter a start date and an end date, and you get the total in days, weeks, months, and years – with optional business-day filtering.
How does a date duration calculator work?
A date duration calculator measures the interval between two calendar dates. Internally, each date is converted to a serial number (the number of days since a fixed reference point), and the difference between the two serial numbers gives the duration.
Example:
| Start date | End date | Duration |
|---|---|---|
| Jan 1, 2026 | Dec 31, 2026 | 364 days |
| Jun 1, 2025 | Jun 1, 2026 | 365 days |
| Feb 1, 2024 | Mar 1, 2024 | 29 days (leap year) |
The calculator automatically handles:
- Unequal month lengths – 28, 29, 30, or 31 days
- Leap years – an extra day added to February in years divisible by 4 (except century years not divisible by 400)
- Cross-year intervals – spanning December into January
Calendar days vs. business days
Calendar days include every single day – weekdays, weekends, and public holidays. Business days (also called working days) count only Monday through Friday, typically excluding national holidays.
This distinction matters in practice. A 60-day legal notice period contains roughly 42–44 business days, depending on which holidays fall within the span. Contract deadlines, shipping estimates, and payroll calculations all rely on business-day counts.
When using the date duration calculator above, you can switch between calendar-day and business-day output to match your needs.
How to calculate the number of days manually
If you need to verify a result or work without an online tool, follow this approach:
- Write down both dates in the format year–month–day (e.g., 2026-03-10).
- Count the remaining days in the start month (from the start date to the end of that month).
- Count full months between the start and end months, adding up the days in each.
- Add the days in the final month (from the 1st to the end date).
- Sum all three parts.
Worked example – from April 20 to August 5:
- April: 30 − 20 = 10 days remaining
- May: 31 days
- June: 30 days
- July: 31 days
- August: 5 days
- Total: 10 + 31 + 30 + 31 + 5 = 107 days
This method works for any pair of dates, but a calculator eliminates the risk of arithmetic errors.
Converting days to other time units
The date duration calculator displays the result in multiple units at once. If you are converting manually, use these multipliers:
| From days to | Multiply by |
|---|---|
| Weeks | ÷ 7 |
| Hours | × 24 |
| Minutes | × 1,440 |
| Seconds | × 86,400 |
Example: 107 days = 107 ÷ 7 ≈ 15.3 weeks = 2,568 hours = 154,080 minutes.
For months and years, a direct day-count conversion is approximate because months vary in length. The average month is 30.44 days; the average year is 365.25 days (accounting for leap years). The calculator computes exact month-and-year intervals by following the calendar structure rather than using averages.
Common use cases for date duration calculations
- Project management – measuring sprint lengths, milestone gaps, and total project timelines
- Age calculation – determining a person’s exact age in days from their birth date
- Contract and lease tracking – counting how many days remain until a lease or agreement expires
- Pregnancy tracking – calculating gestational age in weeks and days
- Legal deadlines – verifying compliance with notice periods and filing windows (e.g., a 90-day statute of limitations)
- Event planning – counting days until weddings, conferences, or product launches
- Financial calculations – determining the exact number of days for interest accrual on deposits and loans
Leap years and their impact on duration
A leap year contains 366 days instead of the usual 365, with February 29 added to the calendar. Under the Gregorian calendar:
- A year is a leap year if it is divisible by 4
- Exception: century years (1900, 2100) are not leap years unless divisible by 400
- 2000 was a leap year (divisible by 400); 1900 was not
If your date range includes February 29, the total day count will be one day higher than the same range in a non-leap year. For instance, January 1 to March 1 is 60 days in 2026 but 61 days in 2024.
The date duration calculator accounts for leap years automatically, so you never need to adjust manually.
Accuracy tips
- Specify the year – “March 5 to November 12” yields different results depending on the year, mainly because of leap years.
- Decide on inclusivity – some contexts count both the start and end dates as part of the duration (inclusive counting adds one day). The calculator above measures the interval between the two dates, not including both endpoints. If you need inclusive counting, add one day to the result.
- Account for time zones – date-only calculations ignore time zones. If hour-level precision matters, factor in the UTC offset of each location.
This tool is for informational purposes. For legally binding deadlines, verify dates with the relevant authority or legal counsel.