The Reveal that Ophiuchus used to be a Gold Cloth and the thirteenth Gold Saint is supposed to herald the end of Sanctuary and the world sheds much light on both Shaina's initial Ax-Crazy characterization and her Character Development:
The legend of the Thirteenth Gold Saint is well known and it's not exactly difficult to guess which constellation it would be supposed to match (while there's other constellations that cross the ecliptic, only Ophiuchus was actually included in the earliest Zodiacs, and anyway a walk to the middle point between the Scorpio and Sagittarius temples will show there used to be a building there), Shaina was likely ostracized and treated as a potential traitor until she snapped and decided to act like it.
When Shun fights the Cancer saint of the past, he's sent to the underworld and bears witness to the souls of the dead wandering into the afterlife, and amongst them he recognizes Seiya. Who at this point in time doesn't even exist yet. Heck, if you prefer the reincarnation theory to the Generation Xerox pattern Next Dimension establishes, then Seiya's soul is currently Tenma, who is alive and well.
For whatever reason, the gang only has three days to destroy Hades's sword in the past and save Seiya.But they're in the past, so they'd technically have until Hades dies and reincarnates.
Then again Chronos is trolling them so it is likely he has his own reasons for this restriction.
