I like to think of it as a similar universe to the one used in the Kingdom Hearts games, where all the worlds are connected to each other on a sort of network accessible by Gummi ship. In my head, each FF game takes place on a different world and these are connected and accessible too (which would explain things like Zidane having once known a 'spiky-haired dude') although the method of travel is never discussed. As for time periods, that is open to certain amount of interpretation.