How exactly do ships figure out the correct date after exiting the warp? Do they have to get it from an external source or do they have some way to figure it out on their own?

For Black Crusade when my players do the Exile escape route out of the vortex. Big scene taking the pilot on board to help the navigator, wailing and knashing of teeth, slaves exploding warp fuckery then them trying to figure out where and when they've been spat out into real space