The exact number of days between any two dates
The exact number of days from one calendar date to another, plus the same gap expressed in weeks, months, hours and minutes. Put a past date in and you get how long ago. Put a future date in and you get the countdown. It handles dates either side of leap years and century boundaries without drifting.
This is where most date disputes come from. From the 1st to the 5th is four days if you count the gaps, five if you count the days themselves. Neither is wrong — hotels bill the gaps, holiday allowances usually count the days. Contracts and deadlines almost always mean the gap, so that is what this tool returns. If the thing you are calculating counts both ends, add one.
A month is anything from 28 to 31 days, so "three months" from 30 November lands somewhere fuzzy in February. Any month figure here is an approximation over the real day count, and the day count is the number to trust when something depends on it.
Everything runs in your browser. The dates you type are never sent anywhere.