639 years? Big deal, The Long Now foundation built a clock to last 10,000 years. I hate John Cage since I got his massive world-wide hit 4′33″ stuck in my head.
I think at one point the Van Horn TX clock was considered a "prototype" or another one that would be built incorporating lessons learned, although I don't know if that's still the plan.
Coincidentally the clock will ring with a cycle of chimes that repeats every 10,000 years