Whether you're counting down to a holiday, measuring how long a project ran, or comparing two events, you need to know how many days are between two dates. It sounds simple, but leap years and varying month lengths can trip you up. This guide explains the logic; for an instant answer, use the date difference tool.
How it's calculated
The core idea is to convert both dates to a day count and take the difference. Doing it by hand, watch three things:
- Month lengths: months range from 28 to 31 days, so "3 months = 90 days" isn't always true.
- Leap years: years divisible by 4 (except those divisible by 100 but not 400) have 366 days, with 29 February.
- Is the start day included? Usually the difference is end − start; if you also want to count the first day, add 1.
In days, weeks and months
The same gap can be shown in different units: total days, whole weeks (days ÷ 7), or a years-months-days breakdown. The tool shows them all at once, and can also count only business days (weekdays) — handy for project and deadline planning.
What it's good for
Age calculation, days until a birthday, contract and subscription lengths, pregnancy tracking, leave planning and deadlines all rely on the gap between two dates. To measure from a birth date to today, use the age calculator; to count down to a target, use the day counter.
In short
The days between two dates come from converting each to a day count and subtracting; leap years and changing month lengths make it awkward by hand. For an accurate answer in seconds, try the date difference tool.