NaN javascript error when calculating between dates with timestamp

I would like to begin by saying i looked at multiple threads in this forum before posting. Wasnt able to find my solution :(

Issue: getting a NaN error when trying to find the difference between two dates with a timestamp from two textboxes.

The date format i'm using is DDMMYYYY HH:MM - 27/01/2015 00:00 code below.

thank you in advance for this super helpful forum :)

function stringToDate(s) {
    var dateParts = s.split(' ')[0].split('-'); 
    var timeParts = s.split(' ')[1].split(':');
    var d = new Date(dateParts[0], --dateParts[1], dateParts[2]);
    d.setHours(timeParts[0], timeParts[1], timeParts[2]);

    return d;
}

function test() {
    var a = textbox_1.value;
    var b = textbox_2.value;
    alert(stringToDate(a) - stringToDate(b));
}