Board Thread:Naruto Discussions/@comment-26508646-20150704031517/@comment-6769189-20150715113442

Like I said. At the beginning of their conversation, Hagoromo implies his life force is still unstable, meaning he didn't have the tailed beast to keep him alive.

Itachi was noted to be on an advanced level when it comes to genjutsu. Considering how long Guy spent fighting I would say there wasn't any time dilation.