By default, works for timeline "update" mod and "post" mod, so check the field should be there by default when creating an update or post. Nevertheless, Check forms in your admin studio, activate the field "publishing time" if it's deactivated.

Best use studio->developer->forms->post->create post/edit post/view post

Check for each to activate "publishing time" in all views (i.e do for create post, edit post and view post)