Counting days between two dates sounds like simple subtraction, but months of different lengths, leap years, and the “do I count both ends?” question all conspire to give you an off-by-one answer. Here's how to do it right, plus a free date calculator that removes the guesswork.
The counting method
The reliable way is to count whole months between the dates, convert to days using each month's actual length, then add the leftover days — remembering February has 29 days in a leap year. Because that's fiddly by hand, most people either count on a calendar or let a tool do it. A leap year is any year divisible by 4, except century years not divisible by 400 (so 2000 was a leap year, 1900 wasn't).
Inclusive vs exclusive: the off-by-one trap
The single biggest source of confusion. From 1 March to 5 March:
- Exclusive (the gap between them) = 4 days.
- Inclusive (counting both the start and end day) = 5 days.
Neither is “wrong” — it depends on the question. A hotel stay counts nights (exclusive); an event lasting “from Monday to Friday” counts days (inclusive). Always decide which you mean before you count.
Counting only working days
For deadlines and delivery estimates you often want business days, not calendar days — excluding weekends and public holidays. A rough rule: total days × 5/7 approximates working days, but holidays make an exact count worth using a tool for. The date calculator can count the plain gap; subtract known holidays for a true working-day figure.
Where it's used
Notice periods, project deadlines, loan and interest periods, visa durations, countdowns to an event, and age gaps. For an exact age in years, months and days from a birth date, use the age calculator instead.
FAQ
Should I count both the start and end date? Only if you want an inclusive count. For the gap between them, count exclusively.
How do leap years affect it? They add a day (29 Feb) to any span crossing that date, so a good tool accounts for them automatically.
How do I count working days? Exclude weekends and holidays from the total; roughly 5 of every 7 days.
Is my data private? Yes — the calculator runs in your browser and stores nothing.