<script>

    getHourSubHour('13:30', '17:59');

    function getHourSubHour(hs, he)
    {
        var sHour1 = he.split(":");
        var sHour2 = hs.split(":");
        var iSeconds1 = 0;
        var iSeconds2 = 0;
        var iSeconds3 = 0;

        sHour1[0] = parseInt(sHour1[0]);
        sHour2[0] = parseInt(sHour2[0]);

        sHour1[1] = parseInt(sHour1[1]);
        sHour2[1] = parseInt(sHour2[1]);


        iSeconds1 = (sHour1[0] * 3600);
        iSeconds1 += (sHour1[1] * 60);

        iSeconds2 = (sHour2[0] * 3600);
        iSeconds2 += (sHour2[1] * 60);

        iSeconds3 = (iSeconds1 - iSeconds2);
        GetMakeTime(iSeconds3);

    }

    function GetMakeTime(seconds)
    {

        var
        iHours = 0,
            iMinutes = 0,
            iSeconds = seconds,
            sString;

        while (iSeconds > 59)
        {
            iSeconds -= 60;
            ++iMinutes;
        }

        while (iMinutes > 59)
        {
            iMinutes -= 60;
            ++iHours;
        }
        if(iMinutes < 10)
        {
            var sstr1 = '0' + iMinutes;
        }
        if(iHours < 10)
        {
            var sstr2 = '0' + iHours;
        }
        sString = sstr2  + ':' + sstr1;
        alert(sString);
    }
</script>