function not properly processing form data in Google Apps Script

Working on a Google Sheets Script I created that has stopped working.

It appears that the form may not be feeding the function properly, but I’m not sure why.

This part of the script is intended to be sure that the user has entered a user ID and an item ID for checkout.

form:

 <div class="inline form-group">
   <label for="uid">User ID</label>
   <input type="text" name="chkOutUID" id="uid" value="" size="6"></div>
   
   <div class="inline form-group">
   <label for="iid">Item ID</label>
   <input type="text" name="chkOutIID" id="iid" value="" size="6"></div>

function:

    function confirm(uid,iid,datepicker) {
    if((uid=="" && iid=="") || (uid=="" || iid=="")) {
    ui.alert("You must enter User ID and Gear ID");
} else {

var userID=uid.toString();
var itemID=iid.toString();
var enteredDate = new Date(datepicker);
var dueDate;
//if(datepicker="click to choose date") {
if (Object.prototype.toString.call(enteredDate) !== "[object Date]") {
    Logger.log("is not a date");
    var defaultDue = new Date();
    var defaultDueDate =defaultDue.getMonth()+1+"/"+defaultDue.getDate()+"/"+defaultDue.getYear();
    dueDate=defaultDueDate;
    } else {
    var enteredDue = new Date(enteredDate);
    var dueDate = enteredDue.getMonth()+1+"/"+enteredDue.getDate()+"/"+enteredDue.getYear();
    }
}

Error called is Type Error: Cannot call method “toString” of undefined