JSFiddle - React, Tailwind, and code Playground

HTML

Severity: <br />

<select name="sever">
    <option>Please Select Severity</option>
    <option value="1" data-hours="12">Critical</option>
    <option value="2" data-hours="24">Major</option>
    <option value="3" data-hours="36">Minor</option>
</select><br />

Expected Completed Date: <br />

<input name="exc_date" style="width:150px" type="text" />

JavaScript

$('[name="sever"]').change(function() {
    var h = $(this).find('option:selected').data('hours');
    var date = $('[name="exc_date"]');
    
    if (h == null) {
        date.val("");
    } else {
        var time = new Date();
        time.setHours(time.getHours() + h);
        date.val(time);
    }
});