Transparent method
The formula
Worked example
Example inputs
How to interpret the result
A date difference spanning a leap year needs that extra day accounted for, or the day and week count comes out one short, since February's additional day in a leap year is a real calendar day sitting between two arbitrary dates, not an adjustment that can be skipped without changing the actual count.
Before you rely on it
What to check
Confirm whether any leap year falls within the counted date range, since that adds a real day to the total that a simple 365-day-per-year assumption would miss.
The common error
Where people go wrong with date difference calculator
Estimating a date difference using a flat 365-day-per-year assumption across a range that spans a leap year. That undercounts the true difference by one full day for each leap year crossed.
Sensitivity evidence
How start date changes the days between
| Start date | Days between | Weeks between |
|---|---|---|
| December 18, 2025 | 253 | 36.1 |
| December 25, 2025 | 246 | 35.1 |
| January 1, 2026worked example | 239 | 34.1 |
| January 8, 2026 | 232 | 33.1 |
| January 15, 2026 | 225 | 32.1 |
Every input, tested
Which input moves the days between most
| Input | Tested from | To | Days between at each end | Swing |
|---|---|---|---|---|
| Start date | December 25, 2025 | January 8, 2026 | 246 to 232 | 14 (5.9%) |
| End date | August 21, 2026 | September 4, 2026 | 232 to 246 | 14 (5.9%) |
Two variables at once
Days between by start date and end date
| Start date \ End date | August 14, 2026 | August 28, 2026 | September 11, 2026 |
|---|---|---|---|
| December 18, 2025 | 239 | 253 | 267 |
| December 25, 2025 | 232 | 246 | 260 |
| January 1, 2026 | 225 | 239 | 253 |
| January 8, 2026 | 218 | 232 | 246 |
| January 15, 2026 | 211 | 225 | 239 |
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Start date | January 1, 2026 | |
| End date | August 28, 2026 | Defaults to today. |
| Days between | 239 | |
| Weeks between | 34.1 | |
Inputs, definitions and assumptions
Start date
End date
Defaults to today.
Where the inputs come from
Enter the start date and the end date of the interval you want to measure. The tool counts the days between them and converts to weeks. Decide whether you want to count the end date as well: this calculation subtracts one date from the other, so the day count excludes the start date, which matches how most people count the time between two dates. Check the year on both dates, especially across a leap year.
When the result stops applying
Counting conventions vary: contracts may include or exclude the first and last day, and some legal periods count the last day only if it is a business day. Leap years add a day in February. Time of day is ignored. Months have different lengths, so converting days to months is approximate.
What to compare it with
Count the weekdays on the business days calculator, find a future date on the add days to date page, and count down on the day counter. The day of the week calculator identifies the weekday, and the age calculator counts years. Check the contract for how the period is defined.
People also ask
Frequently asked questions
Does the day count include the start date?
This calculation subtracts one date from the other, so it does not count the start date. Add one if you want both the first and last day included.
How do I count business days between two dates?
Use the business days calculator, which excludes weekends from the count. Public holidays need to be subtracted separately because they vary by location.
Why does a period of 30 days differ from one month?
Months vary from 28 to 31 days, so a month is not a fixed number of days.