Calculating difference between two dates using JS

[yotamaner]

Hi,
I saw that someone asked a similar question before, but it's not what I'm looking for.
I have two date fields (check in, check out) and "number of days" field.
I want the script to calculate automatically the difference.
For example: I have defaults dates, and I want the script to put the difference in "number of days" field. And if the user will change the date, the number of days will change automatically.


Thanks,
Yotam

[doktora]

Have you tried this:

var numdays = Number(10);
var now = new Date();
var before = new Date();
before.setTime( now.getTime() - numdays * 24 * 60 * 60 * 1000);