User blog comment:B214/Volume 21 info & spoilers/@comment-4547267-20160319075454/@comment-3965848-20160320180551

At the end of Ex, future Gasper tells Azazel about when the UL will hit and to prevent the war, so there won't be a need to travel back or a war in the current canon timeline.

And Issei's kids might be born in differen order with different names, the ones that appered in EX now exist in an alternate future earth, hence cross-time travel, which is traveling to points in time on other earths.