Jack Cole, the Playboy cartoonist, must have been a little boy when he wrote the adventures of Boy Inventor Dickie Dean. Dickie’s inventions included a machine to capture conversations from the past (Silver Streak Comics 3), a time camera (probably in issue 10). You could argue that neither of these is real time travel, but never mind.

I’ll bet there was more time travel in various of the comics published by Lev Gleason, but I haven’t yet tracked them down.