Computed field module: value of field when creating new content
menu_get_object() provides access to objects loaded by the current router item. For example, on the page node/%node, the router loads the %node object, and calling menu_get_object() will return that.


From: http://api.drupal.org/api/drupal/includes!menu.inc/function/menu_get_object/7

It returns empty upon creation because you add the node from "node/add/[content-type]" page but update it from "node/[nid]" and it can use the nid to load the node.

If you need just to duplicate the value from the start_time field of the same node, use this:

$entity_field[0]['value'] = $entity-&gt;field_start_time[0]['value'];