First OED2 citation of the “ahead” version, in the sense of having ideas, etc., that are not accepted in one’s own time but will come to be later, is from George Bernard Shaw’s “On the Rocks”, 1934: “Women and men who are ahead of their time. They alone can lead the present into the future. They are ghosts from the future.”
“Before his time” is usually used in other senses, typically indicating either prematurity ("he died before his time") or just chronological separation ("Shakespeare never met Cleopatra, she was before his time.") What sense are you interested in?
