The xerxes is my private unit of chronometry. One xerxes separates year A and year B such that every adult alive in year A is dead by year B. The xerxes, is only an approximation. It excludes outliers—the lapse of a xerxes does not wait on the expiration of the world's last living; it excludes anyone whose very survival is newsworthy. A xerxes is concluded once you can look at a picture or a list of names and be confident that all the faces and all the names you see belong to dead men. Setting adulthood at 15 and lifespan at three score and ten, and affording a margin of safety of 1/3, a xerxes rounds down to a period of 70 years.

The xerxes implies another unit, the half-xerxes. The half-xerxes separates year A and year B such that half of all people alive in year B were not yet adults in year A. I set the half-xerxes, with less certainty, at 35 years—with less certainty because wars, plagues, and baby booms too easily throw it off. (Otherwise we could more elegantly rely on the half-xerxes as our primary unit.)

Nothing happens only when it happens. Everything memorable recurs in memory until memory is extinguished. And when that memory is a shared one, like the memories enforced by disaster and strife, it establishes, among those who remember, a secret language of allusions and reminders with power beyond ordinary language. The consequences of this language, even at dark removes, are still in truth the consequences of that event. Thus in order to judge an event, we must look at it three times: first, contemporaneously, so we can judge what it means when everyone remembers it; second, from a half-xerxes, so we can judge what it means once most people no longer remember it; and third, from a xerxes, so we can judge what it means once no one remembers it. And until the third look all judgments remain provisional.

Consider the world wars. Given the double carnage of the trenches and the Spanish Flu, it is plausible to mark a half-xerxes between the end of WWI and the beginning of WWII. And though a connection is difficult to assert it is suggestive that the Soviet Union, instituted in reaction to WWI, collapsed a xerxes after it. In Russia and Europe, where WWII was bloodiest, the half-xerxes of that war perhaps came in 1968. In the US it took the full 35 years, until 1980 and the collapse of the "liberal consensus." (On this pattern I intend to keep my eyes open in 2015, when the xerxes of WWII concludes.)

I take the name of this unit from a story in Herodotus. Xerxes the Persian, king of kings, looked over his army of a million men, the greatest army the world had ever seen, absolutely loyal to him, he the greatest ruler the world had ever known—a million men aimed under his command to the ruin and conquest of obstreperous Greece. But as he sat and saw on his hillside throne, something gave way in his mind. Some inward support, rotted out with secret melancholy, broke and let him fall. Xerxes looked at a million strong, proud, fearless men; but Xerxes saw only time and decay and death. Xerxes (a poor calculator) thought that in a hundred years not one of these men would be alive; and thinking so, Xerxes, king of kings, before his army of a million men—Xerxes wept.