Updated August 26, 2026
Date difference units reference
Measuring the time between two dates comes down to counting whole calendar days and grouping them. This sheet lists the common conversions, the two ways to count endpoints, and how leap years affect the total.
Common conversions
| Unit | Days |
|---|---|
| 1 week | 7 days |
| 1 fortnight | 14 days |
| 1 month (average) | ~30.44 days |
| 1 common year | 365 days |
| 1 leap year | 366 days |
| 1 year (average) | 365.25 days |
Counting endpoints
| Mode | What it counts | 1 Jan to 3 Jan |
|---|---|---|
| Exclusive | The gap between the dates | 2 days |
| Inclusive | The gap plus the start date | 3 days |
Weeks and remaining daysDivide the total day count by 7 to get full weeks; the remainder is the leftover days. For example, 20 days is 2 full weeks and 6 remaining days.
Months and years varyMonths range from 28 to 31 days and years from 365 to 366, so day-based averages are approximate. For an exact count between two specific dates, count calendar days directly rather than multiplying by an average.
References
Questions
How many days are in a month?
It varies from 28 to 31. The average across a year is about 30.44 days, but for an exact date difference you should count calendar days rather than use the average.
Should I count dates inclusively or exclusively?
Use exclusive counting to measure the gap between two dates, and inclusive counting when the start day itself should be counted — such as billable days or nights on a trip.