var i;
    var dates = document.getElementById("event_date")
    for (i = 1; i < dates.options.length; i++) {

    var dateString = dates.options[i].value;
    var date = new Date(dateString)
    var timestamp = date.getTime();
    var anfangsdatum = new Date("2019-06-01 23:59:00").getTime();
    var enddatum = new Date("2019-07-01 23:59:00").getTime();

    if (anfangsdatum < timestamp < enddatum ) {
    // The selected time in summer
    dates.options[i].style.display = "block";
    }
    else if (timestamp > enddatum) {
    dates.options[i] = null;
    }
    else {
    dates.options.remove(i);
    }
 }