Let’s fast-forward a bit. Bob goes from a virile young man to a feeble old one over the course of many, many years — and many, many big round rocks, as Bob was prone to messing up on the calendar and did not enjoy today’s luxury of an “Undo” command — all the while carving out the calendar that he had been given the nearly-holy task of creating. Why, he's been at it so long, the very men who bestowed the honor of creating and completing the calendar have all since passed on. Essentially, Bob is now very much akin to a man who started out in the mail room of a big business, moved his way up to the top, and later became the store’s elderly janitor: everyone sees him, but has no idea that's he's there.
Finally, one day, after many years of toil, trial and error (not to mention all of those big round rocks), Bob finishes his assigned project — before keeling over dead. He has died from natural causes, having worked on the calendar almost all of his life. Unfortunately, the younger generation that came to follow Bob’s could never fathom his antiquated and confusing method of calendar-carving — much in the same way that Bob could never figure out why all the young Mayan boys wore feathers in their hats.
Since no one else has so much as an inkling of a clue as to what they should do with Bob’s calendar, they wheel it into the “Old Shit We Can’t Figure Out” exhibit at the local museum, much like we see VideoDisc and top-loading Betamax players in today’s antique shops. Eventually, the Mayan people begin to dissipate, disappear and/or dissolve into the stories of legend (only to have people refer to them as “the Aztecs” a millennium-and-a-half later). When the Spanish came-a-marchin’ on in many moons after Bob’s demise, they discovered a strange circular object and said "Que es esto?" — unawares that this impressive object was once the very coal that burned Bob’s fire of life for so many years. Instead, the Spaniards used it as a big-ass coffee table.