For the 'Trousers of Time' (god that's a great name) Event, Diarmuid has two 'worlds' he could kind of call his own, namely because he has lived in two vastly different times. Would any tears to his world only go back to Japan and the time of the Grail War (which is when he was pulled from) or would it be possible to have a tear go back to when he originally lived in Ancient Ireland as well?