User blog comment:Jho5689/Fairy Tail 337 and onwards/@comment-11232389-20130625163146/@comment-7418318-20130626093946

They believed that they can't kill Zeref in the present, because he's too powerful, so they build the Eclipse Gate to send someone 400 years into the past and kill Zeref before he became imortal.

But when they opened the gate the 10.000 Dragons came through and devastated the world.

Even though F-Lucy didn't know that the Dragons came through the Eclipse Gate, her presence in the second timeline (the one F-Rogue came from) either prevented the opening of the Gate or caused the Gate to be closed before any Dragons came through, but at some unspecified later point Acnologia came out of hiding and devastated the world on it's own.

In order to slay Acnologia F-Rogue invented Dragon Manipulation magic, but since there are no Dragons in his time, he had to travel back 7 years.

I suspect this 7 year timejump was necessary because the gate didn't have enough power stored to open a portal to 400 years ago from his own time, so instead he went to the present and manipulated Hisui into opening that 400 year "time-tunnel" for him.