JSFiddle - React, Tailwind, and code Playground

HTML

<form ENCTYPE="multipart/form-data" ACTION="servlet1" METHOD=GET>
    <center>
    <fieldset>
            <label for="Start_Date">Start_Date</label>
            <input type="date" id="Start_Date" name="Start_Date" > <br> </br>
            <label for="END_Date">END_Date</label>
            <input type="date"  name="End_Date" id="End_Date"> <br> </br>
              <input type="submit"  value="Submit" onclick="return check_date();"> </input>
    </fieldset>
    </center>
  </form>

JavaScript

function check_date() {

        // start_date and End_date now point straight at the correct boxes.
        var start_date = document.getElementById("Start_Date");
        var End_date = document.getElementById("End_Date");
    
        // convert both entries to a Date object from the string value
        var startdate_2 = new Date(start_date.value);
        var enddate_2 = new Date(End_date.value);
    
        alert(startdate_2.getTime());
    
        var diffDays = (enddate_2 - startdate_2)/(24*3600*1000);
        if ( diffDays > 14 )
        {
            alert("You entered an invalid date. Please repeat again");
            return false;
        }
        else
        {
            alert ("Will Proceed");
            return true;
        }
    }