Possible to edit custom date field and display?

Editing the theme is a good way to get this done, if it’s a theme with updates from someone else I would make a child theme to prevent your changes being overwritten:
https://developer.wordpress.org/themes/advanced-topics/child-themes/

Another option is to make a plugin that utilizes a shortcode that allows any WordPress editor to just plug that shortcode into the WordPress page editor. There might be a plugin that already does this, search?

Wherever you need the date changed you could add this code in replacement:

<?php
$acfCustomDate = strtotime(get_field('custom_date'));
$currentDate = strtotime(getdate());

$secsAgo = $currentDate - $acfCustomDate;// == strtotime gives seconds
$daysAgo = $secsAgo / 86400;

if($daysAgo > 0)
    echo "Posted $daysAgo days ago";
else
    echo "Posted $daysAgo days in the future!";