If you want to calculate the number of days between two dates, you need to make a function,pass the dates and use getTime() function.

Below is the example code :

function GetNoDays(date1, date2) {
    var daysIndates = date1.getTime() - date2.getTime();
    //store the getTime diff 
    return (daysindates / (24 * 60 * 60 * 1000));
    //Convert values to -/+ days and return value
}

var d = new Date(2012, 6, 11);
var d1 = new Date();
var day = d1.getDate();
var month = d1.getMonth() + 1;
var year = d1.getFullYear();
var curntdate = day + "/" + month + "/" + year;
var curDate = new Date(year, month, day);
alert(GetNoDays(curDate, d));

First convert both dates then to obtain the number of days for a given number of milliseconds, we would divide it by 86,400,000, the number of milliseconds in a day (1000 x 60 seconds x 60 minutes x 24 hours).

You can also use moment.js. For this the code looks like this:

var a = moment('1/1/2012', 'DD/MM/YYYY');
var b = moment('1/1/2013', 'DD/MM/YYYY');
var days = b.diff(a, 'days');