why I am getting NAN in safari browser?

I am trying get difference of two time in safari browser .It is working fine in chrome .But on safari I am getting NAN.when I run application I save today date first time.and on second time I run my application I get the difference of save date and now date .it give correct in chrome .but fail in safari why ? here is my code http://ift.tt/1KNZ7nL

var todaydate = new Date();
var datetime = todaydate.getMonth() + 1 + "-" + todaydate.getDate() + "-" + todaydate.getFullYear() + " " + todaydate.getHours() + ":" + todaydate.getMinutes() + ":" + todaydate.getSeconds();

if (localStorage.getItem("getSinkdate") == null || localStorage.getItem("getSinkdate") == '') {
    alert('empty')
    localStorage.setItem("getSinkdate", datetime);
} else {
    var datetimeLocal = window.localStorage.getItem("getSinkdate")
    var d1 = new Date(datetime);
    var d2 = new Date(datetimeLocal);
    alert((d2.getTime() - d1.getTime()) / (1000 * 3600))
    alert((d2.getTime() - d1.getTime()) / (1000 * 3600) > 12)
    if ((d2.getTime() - d1.getTime()) /(1000*3600)> 12)
    {
        alert((d2.getTime() - d1.getTime())/ (1000 * 3600)); 
        localStorage.setItem("getSinkdate", datetime);
    }
}

via Chebli Mohamed