For those of you who do not know, space and time co-exist according to science. MInato has the ability to move from space to space whenever he wants to in an instant, meaning from Konaha to the Hidden Mist whenever he feels. Considering that space and time is the same thing, this means that Minato also has the ability to move through time from one period in history to another period in history, to the past or future, just as easily as he would be able to teleport back to Konaha from a mission. All Minato has to do is place his seal in a certain area and he will be able to use his space-time jutsu to teleport to that specific location in that specific time period he wants to arrive in. This means that a Minato from the past is able to appear in the manga today. If Obito's jutsu is anything like Minato's then he too is able to travel through time but without the use of any seals.