What it is
The Days Between Dates Calculator counts how many days separate two dates, and also shows the gap in weeks, months and years.
Why counting by hand goes wrong
Because months are not the same length and February changes size. Counting on a calendar works for a week and stops working for anything longer, and the errors are never obvious: they are off by one or two days, which is exactly the amount that matters for a deadline.
Leap years are the other trap. 2024 had a 29 February, 2100 will not despite being divisible by four, and any count spanning those dates by hand is one day out.
What people use it for
Deadlines and notice periods. Time until a trip, a birthday, an exam or a due date. How long someone worked somewhere, in days. The interval between two events for a report. Age in days, which comes up more often than you would think in medical and legal contexts.
Counting inclusively or not
This is the detail that produces most disagreements. From the 1st to the 10th is nine days if you count the gap between them, and ten days if you count both endpoints.
Neither is wrong, but they answer different questions. "How many days until the deadline" wants the gap. "How many days does the event run" wants both ends included. Contracts and legal deadlines usually specify which, and it is worth checking before relying on the number.
For business days only, skipping weekends, use the business days calculator.