What is the earliest time one can daven Maariv before sunset?
Answer:
The earliest time for Maariv is after Plag Hamincha. Ideally, Maariv should be davened after nightfall.
see: http://www.myzmanim.com/day.aspx?askdefault=1&vars=39158955&q=johannesburg
Sources:
See Shulchan Aruch , Orach Chaim 233:1