The most remarkable fact about the sexagenary cycle is not its structure but its continuity. Stem and branch day names appear on the Shang oracle bones around 1200 BCE, already in routine use for dating divinations, and the count of days has run unbroken from that horizon to the present: the cycle recorded on a bone unearthed at Anyang connects by simple counting to the cycle printed in this year's almanac. That makes it among the oldest continuously running human reckonings of time. The application to years came later, entering general use under the Han, and the traditional claim that the cycle began in 2637 BCE, when the Yellow Emperor's minister devised it, is legend rather than history, the kind of founding attribution Chinese sources routinely supply; the atlas records it as such.
From China the cycle travelled with the calendar to Korea, Japan and Vietnam, where it named years, fortunes and generations; in Japan the sixtieth birthday, kanreki, is still marked as the completion of a person's own cycle, the return of their birth pair. Historical events keep sexagenary names as proper nouns, remembered by their year pair rather than a number. The cycle asks nothing of the sky, no observation, no intercalation, which is exactly why it has never needed correcting and never broken.