Let's suppose that I am the Captain at a space dock sitting in interstellar space. My crew decides to go on a trip in our ship, so they board and set off at .9c.
t = t0 / sqrt( 1 - v^2/c^2 ), t0 = 1, v = .9c, therefore
t = 1 / sqrt( .19 ) = 2.3 (approximately)
So as they speed off, 1...