JSFiddle - React, Tailwind, and code Playground

HTML

<input  type="text" id="appt_start"/>
<input  type="text" id="appt_end"/>

JavaScript

var hash="10:00";
$('#appt_start').val(hash);

var d = new Date("October 13, 1975 "+hash)  
d.setMinutes(d.getMinutes()+15);

$('#appt_end').val(d.getHours()+":"+d.getMinutes());