reversing polarity control

I am trying to hook up a simple 12VDC linear actuator to a timer and need to stroke in (+)in the AM (morning) and out (-) in the evening. I am automating my chicken building to open in the morning and close in the evening. I can only find a two wire linear actuator so must use some sort of polarity switching relay. Is that just a simple DPDT 12VDC relay? For efficiency sake, in between actuations I want the power system to shut down and only power the timer since this is a 12VDC cell/solar power system. I don't understand polarity switching relays. Any help would be very much appreciated.