Pokémon X and Pokémon Y

I don't see it as much of a stretch, Celebi's power is mainly meant only for it to travel between time, not to allow others to go with it and Dialga can only control the flow of time, not travel through it as far as I know.

quite often in the anime and movies, celebi uses its power to bring someone back in time (there was an episode with may). In the Zoruark movie stealing Celebi's powers (or something like that) gave the villain limited sight into the future (which he used to become rich ofc.). In the Movies, Dialga moves Ash and co. back in time a few times before he is KO'd by Arceus.