JSFiddle - React, Tailwind, and code Playground

HTML

<select name="server" id="server">
    <option value="Default">Please Select Severity</option>
    <option data-hours="12" value="Critical">Critical</option>
        <option data-hours="24" value="Major">Major</option>
        <option data-hours="36" value="Minor">Minor</option>
</select>
<input name="exc_date" id="exc_date" style="width:150px" type="text" />

JavaScript

window.onload = function(){
    document.getElementById("server").onchange = function(){
        var timeObj = document.getElementById("exc_date");
        if(this.value != 'Default'){
            var hours = this.options[this.selectedIndex].getAttribute('data-hours');
            
            var now = new Date();
            now.setHours(now.getHours() + (hours*1));
            var nowStr = now.getDate() + "/" + (now.getMonth()+1) + "/" + now.getFullYear();
            timeObj.value = hours + " hours, " + nowStr; 
            
        } else {
            timeObj.value = '';   
        }
    };
};