Maybe Gajeel knows there's a time discrepancy, but he just doesn't care? I don't recall anything being mentioned about Acnologia coming from some other dimension, unless I missed something (first we heard of it, it just showed up out of nowhere and attacked Gildartz, like the giant reptile version of Monty Python's Spanish Inquisition, and then it seemed to be drawn towards Zeref). Personally, I have a theory that Zeref's a Life or Death Dragon Slayer, (given that he absorbs life to produce life, and I recall him having visible fangs like the other Dragon Slayers) and that might have something to do with how he knows Natsu (Grandeen stated that Natsu and Wendy had met before and that it ended poorly, though neither of them seem to remember it - maybe true Dragon Slayers get reincarnated or something?). However, I could be grasping at straws here.

lol Sounds good. What's your username?