JSFiddle - React, Tailwind, and code Playground

HTML

<label id="asterisk">*</label> <label class="description" for="element_2">Event Date:</label><br /><label>Reservations must be made between <b>45-90 days in advance</b> of the event date</label> <br /><input onclick="one()" required="" type="date" oninput="this.className = ''" name="Event-Date" id="datepicker1" min="2019-10-02" />
    <script>if ( $('#datepicker1')[0].type != 'date' ) $('#datepicker1').datepicker();</script>
  <script>
    function formatISOLocal(d) {
      let z = n => ('0' + n).slice(-2);
      return d.getFullYear()+'-'+z(d.getMonth()+1) + '-' + z(d.getDate());
    }
    
     function one() {
      let inp = document.querySelector('#datepicker1');
       let d = new Date(new Date().getTime()+(45*24*60*60*1000));
      inp.min = formatISOLocal(d);
      inp.defaultValue = inp.min;
      d.setFullYear(d.getFullYear() , d.getMonth() + 6);
      inp.max = formatISOLocal(d);
      // Debug
      console.log(inp.outerHTML);
    }
    </script>