How to Calculate the Number of Days Between Two Dates
How many days until your deadline? How long between two events? Counting across months — and leap years — by hand is fiddly and easy to get wrong by a day or two. Here's the reliable way.
The counting method
Add up the remaining days in the start month, the full days in each month in between, and the days into the final month. The classic mistake is being off-by-one: decide upfront whether you're counting inclusively (both end dates) or the gap between them — that one choice changes the answer.
Watch the leap years
Any span crossing February 29 needs that extra day counted. This is exactly why mental shortcuts (like "months × 30") drift off over long periods.
Calculate it instantly
Our free date difference calculator gives the exact number of days, weeks, months and years between any two dates. If one of the dates is your birthday, the age calculator is the specialised version.
Bottom line
Decide inclusive vs. exclusive counting, respect real month lengths and leap years, and you'll get an accurate gap. For anything important — contracts, notice periods, deadlines — let a date calculator remove the off-by-one risk entirely.
