[ringleader]

The following is a template and file change to show signature only, counter only, or both signature and counter depending on which are turned on in the UserCP by the member.

In the postbit template (not tested in postbit_legacy)

FIND

f condition="$post['signature']">
		<!-- sig -->
			<div>
				__________________<br />
				$post[signature]
			</div>
		<!-- / sig -->
		</if>

REPLACE WITH

<!-- sig and countdown mod -->
		<if condition="$post['signature'] AND $counterhtml">
			<div>
				__________________<br />
				$post[signature]
				<br /><br />
				$counterhtml
			</div>
		<else />
			<if condition="$post['signature'] OR $counterhtml">
			<div>
				__________________<br />
				$post[signature]
				$counterhtml
			</div>
			</if>
		</if>
<!-- /sig and countdown mod -->

In the file included with this mod plugins_usercountd.php

FIND AND DELETE

  if ( strlen($post["signature"]) > 0 )
  {

FIND

	 
	  eval('$counterhtml = "' . fetch_template('usercountdown_display') . '";');
	  
	  $sig = $post['signature'] . $counterhtml;
      $post['signature'] = $sig;
	}
  }
}

REPLACE WITH

	  eval('$counterhtml = "' . fetch_template('usercountdown_display')  . '";');
  }
}

Hope this helps someone else.
Took me the good part of an hour to figure out how to do that
I just installed with no problems on 3.6.8