You said that you tried
$expirationtime = get_field("expiration", $post->ID);
If that does not work then there are only a few explanations for why ACF is returning no value.
1) The field does not exist on the post
2) The field has no value
3) The field name you’re trying to get is not the correct one
4) Some other filter somewhere is interfering with ACF getting the field and formatting the value. This last one can be difficult to track down.
