Sounds about right, I assumed it was similar to the sand castle time piece from a while back.
Do you think that… like when President Nixon had a speech prepared for if Apollo blew up or if the astronauts got stranded on the moon, Randall Munroe has planned out special frames for if something goes wrong?
See also http://xkcd1446.org/ for an archive of xkcd's #livedrawing of the event