javascript中遇到Cannot read property 'split' of undefined问题

function NewDate(dateTime) {
            var fulldate = dateTime.split(‘-‘);
            var day = fulldate[2].split(" ");
            var time = day[1].split(":");
            var date = new Date();
            date.setFullYear(fulldate[0]);
            date.setMonth(fulldate[1] - 1);
            date.setDate(day[0]);
            date.setHours(time[0], time[1], 0, 0);
            return date;
        }

            var sttime, edtime;
            var startTime = document.getElementById("start_time").value;
            var endTime = document.getElementById("end_time").value;
            sttime = NewDate(startTime);
            edtime = NewDate(endTime);
           

<input type="text" id="start_time">
<input type="text" id="end_time">

var sttime, edtime;
var startTime = document.getElementById("start_time").value;
var endTime = document.getElementById("end_time").value;
if (!startTime || !endTime) {
     return;
} else {
     sttime = NewDate(startTime);
     edtime = NewDate(endTime);
}