JSFiddle - React, Tailwind, and code Playground

HTML

<li><label>Enter Your Birthday</label></li>
<li><input type="date" name="field1" class="field1"></li>
<li><label for="date">Current Date</label></li>
<li><span class="dateCalc">x</span></li>
<li><input type="button" value="Click" onclick="calcDate()"></li>

<script >
  var d = new Date();
  document.write(d);

  window.calcDate = function calcDate() {
    var bDay = new Date(document.querySelector(".field1").value);
    var eTime = d - bDay; // diffrence in miliseconds (can be mins if birth date is from future)
    
    document.querySelector(".dateCalc").innerHTML = eTime+ ' [ms]';
  }

</script>