This happens post game after the credits roll.
*** Spoiler - click to reveal ***
When you go to leave Arboria you will be directed to investigate some mysterious ruins in the Dundrasil Region.
On finding the Wheel of Time, go to the Lost Land (using the flying whale), located to the north of Yggdrasil.
Go into the Tower of Lost Time and make your way to the altar. Answer 'yes' to a number of "are you certain?" prompts, watch the cut scenes until you wake up and the achievement will unlock