Thanks for your help. I already solved it.

**My solution:** Use this simple bash script to get timestamp from `hwclock -D` command.

    function timestamp_from_rtc_get {	
    	tmp="$(sudo hwclock -D)"  
    	tmp=${tmp##*= } 
    	out=${tmp%% seconds*} 
    	echo $out
    }
    timestamp_from_rtc_get