Since i needed something like this, I found a very simple way to accomplish this:
1. Create a time picker field with return and display format set to H:i
2. If is a front end form hide “now” button with css.
3. use this in your template to get time
<?php
$preptime = get_field('prep_time');
$preparr = explode(':', $preptime);
$hour = $preparr[0];
$min = $preparr[1];
echo '<time content="PT'.$hour.'H'.$min.'M" itemprop="prepTime">'.$preptime.'</time>';
?>