There is no time travel involved anywhere in FFX. Dream Zanarkand is located on Spira as a summon/an aeon, but no one on Spira found it yet... well except for Auron/Jecht/Tidus whom Sin, who knows about DZ, took from and to DZ. At least that's what I gathered.

What I'm more interested in is what happened to DZ after Sin's attack or why Sin attacked it in the first place. I guess Auron explains it a little bit when you talk to him on the airship after entering Sin, but what happened to DZ is never revealed. And why can't I just fly the airship there to see it for myself?