This may get a bit confusing, so bear with me on this...
The ruined Zanarkand is on Spira, 1,000 years into the future, yeah?
Tidus' Zanarkand is a big city, but there doesn't seem to be anything beyond it. Surely, if the two Zanarkands are the same place, just at different times, the area around it would be the same, ie. Tidus would have lived in Spira. This would mean he would have known about places such as Mt. Gagazet and the Calm Lands, as these are very close by, but he doesn't. Are the two Zanarkands the same place, or is Tidus' Zanarkand on a different planet, completely isolated from anything else?
Here's another thing I thought of. During the Macalania kissing scene Tidus says he and Yuna could fly to his Zanarkand on the airship, but how can he if it's 1,000 years in the past?
Does this also mean that Sin can exist in two time zones at once?