JSFiddle - React, Tailwind, and code Playground

HTML

<div class="datetime">datetime</div>
<div class="placeholder">placeholder</div>

<div class="test">test: should be the same width as above</div>

CSS

.datetime, .placeholder, .test {
    background: #EEE;
    margin-bottom: 5px;
    padding: 10px;
}
.test {
    width: calc(100% - 90px);
}

JavaScript

var paxWidth = 90; //$('.pax').width() + 10;
$('.datetime, .placeholder').css('width', '100%').css('width', '-=90px');