Upstream or Downstream

Three friends canoe upstream for 3 hours, then realise their campsite is downstream. They turn around and paddle downstream for 4 hours, arriving at the campsite. The campsite is 20 miles downstream of their original starting point. The next day they canoe from the campsite back to the original starting point (20 miles upstream) and arrive at 17:00. The river flows at a constant 2 miles per hour and the friends paddle at a constant speed relative to the water on both days. At what time did they leave the campsite for the return trip? Give the time in 24-hour format as a four-digit integer HHMM (for example, 0930 would be written 930).