JSFiddle - React, Tailwind, and code Playground

HTML

<input value='0'> hours<br>
<button class='sub' data-num='15'>-15 min</button>
<button class='sub' data-num='5'>-5 min</button>
<button class='add' data-num='5'>+5 min</button>
<button class='add' data-num='15'>+15 min</button>

CSS

* {
    font-size: 125%;
}

input {
    text-align: center;
}

JavaScript

$(document).on('click', '.add', function(event) {
    var input = $('input');
    
    var inputVal = parseFloat($('input').val());
    var addVal = parseFloat($(this).data('num'));
    var newVal = inputVal + (addVal / 60);
    
    $(input).val(newVal);
});

$(document).on('click', '.sub', function(event) {
    var input = $('input');
    
    var inputVal = parseFloat($('input').val());
    var subVal = parseFloat($(this).data('num'));
    var newVal = inputVal - (subVal / 60);
    
    $(input).val(newVal);
});