On Monday, we published a piece on classic rock — about how data analysis is crucial to defining it and which songs it comprises. We also looked at the artists who — at least on classic rock radio stations — were most disproportionately represented by a single song.

But then there’s the question that formed the piece: Why did I hear Green Day on a classic rock station? The simple answer: After some market research, a programmer decided that a sprinkling of “American Idiot” was precisely what the demographic the radio station wanted to hit would want to hear. The more interesting answer: “American Idiot” is probably becoming classic rock, sooner or later and whether I like it or not.

So what, precisely, is the beginning and end of classic rock, as radio currently defines it?

Looking at my data on the release year for each song in the current classic rock playlist, The Beatles, in the early 1960s, mark the earliest rumblings of the genre. Then, in 1967, we see a jump in the number of plays, thanks to The Doors’ eponymous first album, The Beatles’ “Sgt. Pepper’s Lonely Hearts Club Band” and lots of Jimi Hendrix. Six years later, it’s 1973, the biggest year in the current definition of classic rock:

So when does “classic rock” end? That’s the thornier question.

If we’re being very liberal with our threshold, it hasn’t. We saw that a few songs from recent years were played on stations that bill themselves as classic.

If we’re being even slightly stringent with what qualifies, the most recent song with more than 10 spins across the 25 stations I monitored was “Kryptonite” by 3 Doors Down (released in 2000). But even that seems a bit late.

So what about a somewhat inclusive definition centered around the median classic rock song? Of the 1,652 songs for which we have accurate release year data, the median song was released in 1977. If our definition is the middle 90 percent of songs — essentially, cutting out the earliest 5 percent and latest 5 percent — that gives us a range of 1966 to 1995, which is reasonable.

But including the early 1990s makes some people squirm. Of 1,350 spins of songs released in the 1990s, 699 were released in 1990 or 1991 — so maybe that’s our end date? Indeed, 1991 was an important year in rock, according to Eric Wellman, the station director of Q104.3 FM in New York and the classic rock brand manager at Clear Channel. According to Wellman, “1991, as your data actually says accurately, was where it all splinters to hell.”

Before Nirvana, the hair metal of the ’80s, soft rock of the ’70s and pop rock of the ’60s all found a station to cohabit. “Nirvana came along, and we all went, ‘What?’ ” Wellman said.

Nirvana’s entrance splintered rock-and-roll into any multitude of emulators and reactions. Classic rock radio operates on the idea — as we explored in Monday’s piece — that if you ask people what they consider classic, they will tell you and you can play that. But Nirvana muddied the waters.

“The funny thing is, we play Nirvana now,” Wellman said. “There’s only a handful of songs that you’d play on a classic rock station, but the biggest ones, we play. And you know why? Because the baby that was on the cover of ‘Nevermind’ just graduated from college. That album is 23 years old. Guess what? It’s classic.”

So for the time being, if you want a timeline, classic rock is the stuff between The Beatles …

… and Nirvana.

Defining this stuff is tricky, but that’s what data analysis is for. Will classic rock continue to expand? Only time — and of course, observable market forces — can tell.