The Golden Age Flash time travels for the first time in Flash Comics #44 (June 1943). The explanation is ludicrous, but then again, so are comic books in general! 😀
The Flash eventually helps save the Earth of 2443 by bombarding a visiting sun that was over-heating the Earth with “atomic bullets” and using his super-speed as a propeller to float back to Earth. The story is framed as Jay Garrick tells the story at a “Liar’s Club” but later reveals that it is all true.