You can use following javascript function to calculate number of days between two given dates :

function days_between(date1, date2) {
    // The number of milliseconds in one day
    var ONE_DAY = 1000 * 60 * 60 * 24
    // Convert both dates to milliseconds
    date1 = new Date(date1);
    date2 = new Date(date2);
    var date1_ms = date1.getTime()
    var date2_ms = date2.getTime()
    // Calculate the difference in milliseconds
    var difference_ms = Math.abs(date1_ms - date2_ms)
    // Convert back to days and return
    return Math.round(difference_ms/ONE_DAY)
}

Example to use the function is given below:

var no_of_days=days_between('10/04/2011', '10/07/2011');