Because human events aren't typically all that tidy, historians divide them into digestible epochs that begin and end with the reign of some decapitated royal or, more recently, the round numbers of the Gregorian calendar.
Sometimes, though, the dividing lines are bright, and all the work is done for us.
All right: Before I keep slinging terms like "human events" and "epoch," you should know, up front, this column is not about important people. Rather, it's about our culture's pre-eminent party girls.
It so happens that these bright young things have declined and fallen with eerie synchronicity. (Sorry, Mr. Waugh; couldn't resist.)
Heiress Paris Hilton (what it must be like, I wonder, to have no discernible occupation other than "heiress") was just released from a Los Angeles County jail; ostensibly chastened by the experience, she said during her confinement that she felt divinely inspired to change her ways.
Britney Spears, a brief party pal of Miss Hilton's, has undergone a tortuous public meltdown that began, late last year, with an ugly child custody battle and culminated, in February, with the bizarre spectacle of the once-virginal pop superstar shaving off her hair.
Lindsay Lohan, finally, nixed a hotly anticipated bash that had been booked for July 2 at the Las Vegas nightclub Pure. The occasion — her 21st birthday — afforded the party the perfect punch line: her first legal alcoholic drink.
But the singer-actress, in and out of rehab, "is focusing on her recovery 100 percent," according to a spokeswoman.
If the party isn't over for this threesome — well, let's just say the neighbors have called the cops and it won't be long before the paddy wagon arrives.
This isn't to say relapses aren't in the offing, or that old habits of excessive self-revelation and overexposure won't reassert themselves. Most likely, they will.
But it won't be the same next time.
It seems Miss Hilton, Miss Spears and Miss Lohan have tired of themselves — or the parts of themselves that they share with the public, which is seemingly all of them — almost as much as we have tired of them.
Symbolically, at least, millions of overeager gossip consumers and three young women given to self-display have broken the grip of co-dependency, thanks to sheer existential fatigue, the long arm of the law and the care and feeding provided by Hollywood's most popular church — rehab.
This is good news for culture watchers who are given to gloomy predictions that everything is going to the dogs — that we're blind to the sublime because our collective head is buried in the ridiculous — among other distractions — such as cleavage.
This is good news for everyone not named Hilton, Spears or Lohan.
Indeed, the girls' joint demise is a splendid teachable moment for America's most sacred possession, the children: See, it's really not cool to get sloshed in public, drive without a license or forget your underwear. There are consequences, after all. Like jail, and having to wear a wig when you're 25 and cancer-free.
(Say this for Miss Spears, though: At least she took a crack, however capriciously, at family values.)
So as to exonerate ourselves, in advance, of the charge of taking sadistic pleasure in these events, let's note for the record that each of our subjects has a supportive mother and a sizable nest egg.
Miss Hilton will return to ... whatever it is that Miss Hilton does. While in the clink, apparently, she hatched a plan for Paris Hilton-themed playhouses for sick children to collect donated toys. (I know what you're thinking — "how selfless.")
Miss Lohan is under no threat of being drummed out of the movie business — although, tellingly, her too-clever attempt in "Georgia Rule" to parlay her rowdy real-life persona into a fictional character bombed badly at the box office.
Only over Miss Spears does the vocational question-mark loom. It's far from clear that she'll ever fully revive her music career, as evidenced by last month's wheezy and poorly received minitour (during which she sported the aforementioned wig) as well as the delay of plans for a new album.
And, from now on, her (more likely, her handlers') delicate game of PR kabuki, which pop essayist Chuck Klosterman described for panting Esquire readers in 2003 — "She is the naughtiest good girl of all time," he wrote — is completely shattered and impossible to sustain.
Former Spears rivals such as Christina Aguilera and Avril Lavigne, meanwhile, are soldiering on with far less baggage.
It's quite possible none of this has any meta-meaning at all; maybe it's just a case of three marginally talented young women who were ensnared by fame and all the temptations that go with it. They certainly weren't the first.
The timing seems awfully coincidental, though.
Almost like a fairy tale gone frantically wrong.
That reminds me: Can we at least stop calling them princesses?