JSFiddle - React, Tailwind, and code Playground

HTML

Days:<select id="select">
         <option value="150">150 days</option>
         <option value="200">200 days</option>
       </select>
Date1:<input type="date" onchange="setSecondDate();" id="date1">
Date2:<input type="date" id="date2">

JavaScript

function setSecondDate() {
	var days = document.getElementById("select").value;
  var date = new Date(document.getElementById("date1").value);
  date.setDate(date.getDate() + parseInt(days));
  document.getElementById("date2").valueAsDate = date;
}