The problem is that the return formatting you have set is not a valid date for use in strtotime(). Set the return formatting as, the 3rd selection ACF gives you is. Alternately you can get the unformulated value from ACF…. I think that this will return the needed timestamp, but I’m not sure so you’ll need to test it.
$date = get_field('start_datum', false, false); ?>
<span><?php echo date('d', $date); ?></span>
<span><?php echo date('M', $date); ?></span>
