registering the clock time of {HEAD ~ the day} when said change of {TAIL ~ state} occurs at each station; and 
registering the clock time of { HEAD ~ the day } when said change of { TAIL ~ state } occurs at each station ; and
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH