Spring of year 122 is the soonest you can finish. And while the ending may be very appropriate (s/he saves the world with his/her waiting), that sort of blockage can only exacerbate that problem, so I think a different ending could have been envisioned like aiding a dam or launching into space and colliding with an asteroid. That being said, gameplay is well done and the music is excellent. People will complain that you can't move fast - you're a rock on a 400-year deadline. You don't have to move fast!
One final thing that I really enjoyed - the panicking people. I knew they'd panic at first, but originally I thought they'd get used to me. I am of course their benefactor. But once I realize that i'm just this thing that sits around for seasons and moves occasionally to check on construction progress every year or so, it became apparent that they may not fear me, but seeing me move for those 10 seconds would freak anyone out. Well executed game.