JSFiddle - React, Tailwind, and code Playground

HTML

<span class="mytime">1:18</span>
<br/>
<span class="mytime">8:28</span>
<br/>
<span class="mytime">9:38</span>
<br/>
<span class="mytime">10:48</span>
<br/>
<span class="mytime">11:58</span>
<br/>

JavaScript

$(function () {
    $('.mytime').each(function () {
        var timeElements = $(this).text().split(":");
        var theHour = parseInt(timeElements[0]);
        var theMintute = timeElements[1]-15;
        var newHour = theHour - 1;
        $(this).text(newHour + ":" + theMintute);
    });
});