Prophecy is best when it's vague. "The 6th sun will have set and the 7th moon shine in the sky" could mean anything, including:
  • the 7th of any Eorzean month. These occur about every 37 Earth hours.
  • the end of the 6th day of any 7th Eorzean month. These occur about every 19 Earth days.
  • the 7th of any Earth month.
  • the end of the 6th day of July.
and of course:
  • the last sun of the 6th Astral Era sets and the first moon of the 7th Umbral Era shines in the sky (i.e. at night, the start of every day). This occurs whenever SE wants it to. Plus it's a Gods/Twelve thing, so they don't even have to wait until the end of an Eorzean year.
The 7th Umbral Age might happen this summer, or it might happen with the PS3 release or first expansion. It doesn't have to happen soon. SE could string us along with new lore and quests each month until they're ready.

...
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.
Surely some revelation is at hand;
...
-- Yeats, ca. 1920