NEW YORK, NY.- Mention “Happy Days” to TV viewers of a certain age (raises hand), and the first thing they remember might be not an episode or a scene or a catchphrase but a lunchbox. I’m specifically thinking of a cool Thermos-brand one — featuring Henry Winkler as the show’s pop-phenom greaser, Arthur Fonzarelli, aka Fonzie, aka the Fonz — which luckier ’70s kids than I got to schlep their PBJs to school in and which is now in the collection of the Smithsonian.

To remember “Happy Days” is to remember your youth, which was also the function of “Happy Days” when it premiered in 1974. Well, at least it sort of was. Ostensibly, the show appealed to grown-ups who were young during its time period — roughly, the mid-’50s to mid-’60s, over 11 seasons. But some of its most ardent fans were the lunchbox toters toddling down someone else’s memory lane.

Now “Happy Days” is 50 years old. Or is it? Time gets fuzzy when you enter the “Happy Days”-verse. In some ways, the series never ended; it was just handed down through the culture like a vintage varsity jacket. It was repurposed as a nostalgia object by the Spike Jonze video for Weezer’s 1994 single “Buddy Holly.” In 1998, “That ’70s Show” set its own reverie, like “Happy Days,” among a gang of teenage friends in Wisconsin. Last year, that series’ sequel, “That ’90s Show,” created a ’90s version of the ’70s version of the ’50s.

If all this math is too much, all you need to know is that there are only ever two periods in pop culture nostalgia. There is Then (simple, innocent, fun), and there is Now (scary, corrupt, confusing). Eventually, Now becomes another Now’s Then, and the cycle repeats. “Happy Days” was nostalgic because the teenagers weren’t smoking weed. “That ’70s Show” was nostalgic because the teenagers were smoking weed. We rock around the clock and around the calendar, returning ever again to the beginning.

“Happy Days” did not invent this kind of rosy retrospective memory. But as a mass phenomenon, it was the show that split the atom of nostalgia and got us unstuck in time.

The series grew out of a broader 1970s vogue for the ’50s (and early ’60s), which included the musical and film “Grease”; the hits “Crocodile Rock” by Elton John and “Yesterday Once More” by the Carpenters; and the movies “The Lords of Flatbush” and “American Graffiti.” The ’70s also went through a brief phase of longing for the 1930s — that sweet, untroubled decade! — which gave us “The Waltons” as well as a short-lived variety show, also called “Happy Days.”

The creator of the more famous “Happy Days,” Garry Marshall, was asked to develop a period comedy for ABC. (Part of the thinking, he once recalled, was that you couldn’t make an “honest” show about ’70s teens that would pass the network censors.) He based the premise on his own youth, transposing his Bronx neighborhood to Milwaukee.

The show was also part of a larger counterprogramming decision to lean into feel-good, escapist sitcoms to counter CBS’ dominant lineup of socially relevant comedies (“M*A*S*H” on war, “All in the Family” and “Maude” on culture war, “The Mary Tyler Moore Show” on feminism).

In the Cunningham household, mother Marion (Marion Ross) whipped up biscuits and common sense, while father Howard (Tom Bosley) chuckled in his cardigan sweater about the kids these days. Their naive teen son Richie (Ron Howard) and his friends Potsie Weber (Anson Williams) and Ralph Malph (Donny Most) schemed to get their hands on a set of wheels and to “go all the way” with gum-snapping girls. (Richie had a younger sister, Joanie, played by Erin Moran; his older brother, Chuck, played by multiple actors, went the way of Amelia Earhart somewhere in Season 2.) Any hormonal antics usually ended with a wholesome lesson and maybe a milkshake getting dumped in someone’s lap.

“Happy Days,” in a way, was the ’70s using the ’50s to repudiate the ’60s, or at least the divisive aftermath of them that fueled the one-liners in Archie Bunker’s living room. It mythologized Middle America as a decent place where kids necked in parked cars and everyone was essentially good and happy, where family life was a friendly tussle, not a war. The premiere year, 1974, would see Richard Nixon’s resignation and the beginning of the Gerald Ford years. “Happy Days” was a bet that the TV audience wanted a breather from controversy.

And what brings a divided nation of TV watchers together? TV. By 1974, television was finally old enough to become nostalgic for itself.

So “Happy Days” was a show about TV, or at least about the first TV generation. America first met Ron Howard as a child star of black-and-white TV, as Opie Taylor on “The Andy Griffith Show.” The first pilot of “Happy Days” aired in 1972 as “Love and the Television Set” (later retitled “Love and the Happy Days”), an episode of the ABC anthology “Love American Style” about the Cunninghams becoming the first family on their block to get a TV.

(This early version, interestingly, was much more Norman Lear-like than the eventual series. Here, Chuck Cunningham — an amiable jock in “Happy Days” — refuses to say grace at dinner because he is questioning religion, while Howard Cunningham, played here by Harold Gould, is surprised to see “a colored man” — the mail carrier — sitting in his chair at the family’s TV-viewing party.)

The 1950s were when TV started telling American families how to be American families. The story may have been a lie — and may have left out a lot of actual families — but people wanted to believe it.

The Cunninghams of “Happy Days” spend their evenings watching Uncle Miltie and pro wrestling. When Richie wants to buy a used jalopy, Howard points to the Nelson boys of “Ozzie and Harriet”: “David and Ricky don’t have a car.” In the Season 1 finale, Howard decides to build a backyard bomb shelter because he was upset by an Edward R. Murrow special on the Cold War.

The Cunninghams reproduce the ’50s sitcom nuclear family, but they also update it, just a touch, finding a compromise position between the Nelsons and the Bunkers. Marion is a dutiful homemaker, but she speaks her own mind. Howard is an old-fashioned grump, but he’s too nice to ever go full Archie. The kids rebel, a little, but they’re driven by hormones, not ideals. In a second-season episode about the election of 1956, Richie defies his Republican father to support Adlai Stevenson — but only to impress a girl.

Like the past itself, “Happy Days” is better fondly remembered than rewatched. The first few seasons are the show at its best, sweet and fluffy and fun. Winkler, in particular, is astonishing. His early appearances have little dialogue; he does his talking mostly with looks and gestures, like a leather jacket mime.

But as it became a hit, “Happy Days” turned into a crowd-service machine. The evermore popular Fonzie became the Monster That Ate Milwaukee. A Season 5 episode in which he water-ski-jumped over a shark became a metaphor for the point at which a good show goes bad.

Really, there was a whole school of fins in the water. There were melodramatic special episodes. The catchphrases metastasized (“Aaaay!” “Sit on it!”). Marshall, acting on his son’s request that he add an alien to the show, cast the frenetic Robin Williams in a guest spot as Mork from Ork, who would later head up “Mork & Mindy.” (“Happy Days” also spun off “Laverne & Shirley” and “Joanie Loves Chachi,” among several others.)

The series’ legacy, though, is more than the sum of its sharks. It harnessed the cultural pull of memory, and it anticipated how the past would become more inescapable the more of it was captured on video.

Supercharged by TV, American nostalgia developed layers, like a living room wallpapered once a decade. The 2020s look back to the 1990s which looked back to the 1970s which looked back to the 1950s. Of the many reasons that nostalgia never ends, the biggest may be that it is chasing a fantasy, which is always at least one generation out of grasp.

This is also what makes it potent, and not just as entertainment. The 1950s were lionized by conservatives in the Reagan 1980s; the imagined past is what puts the “Again” in “Make America Great Again.” (Nostalgia isn’t automatically reactionary as an art form — “Happy Days” was mildly ’70s centrist, if it had any ideology at all — but it’s a tougher fit with the progressive ideal of society advancing over time.)

The present can’t compete with a memory, especially a selective one. As critic John Leonard wrote, reviewing the “Happy Days” premiere in The New York Times under the pen name Cyclops, “The fifties were also a time when Joe McCarthy’s jugenbund ran around firing the teachers in our schools and the actors on our screens; when Egypt and Israel had another war; when the Soviet Union invaded Hungary; when Adlai Stevenson was ridiculed by Richard Nixon for proposing an end to A-bomb and H‐bomb testing. Happy days!”

“Happy Days,” atypically, once made this point itself — through, of all characters, Mork. In 1979, with “Mork & Mindy” now a giant hit, Robin Williams returned to “Happy Days” for a fifth anniversary special, in which Mork traveled back in time to visit an era “when life was simpler, when things were humdrum.”

Most of the episode is a typical clip special. Mork uses telepathy to summon characters’ memories, in the form of scenes from the sitcom’s first five years. But it takes a turn at the end, when he reports his findings back to his Orkan superior, Orson (voiced by Ralph James).

The “Happy Days” era, Mork says, was “a wonderful, naive and romantic time.” But he adds that, when he reads his hosts’ minds, “They all seem to block out one thing”: McCarthy, the senator from the Cunninghams’ home state of Wisconsin who spent his early 1950s whipping up a red scare in Washington. “I guess that’s why it’s so romantic,” Mork says, “because they never remember the sad things.”

In the five decades since 1974, TV nostalgia has become more plentiful and more complex. “The Wonder Years” was bittersweet (and eventually got its own reboot to diversify and complicate it). “Freaks and Geeks” gave us adolescence as fondly remembered torture. “Mad Men” savored its midcentury swellegance while telling us that nostalgia is “the pain from an old wound” and that the idea of the innocent past is a great American sales job. There is even a kind of TV anti-nostalgia, seen in reclaimed-pop-history stories like “Pam & Tommy,” which flatter audience members with their superiority to the unevolved denizens of the past.

The truth at the heart of all these shows, though, is the same as it was in the days of Richie and the Fonz. Half of what makes our bygone happy days happy is what we remember of them. The other half is what we forget.

This article originally appeared in The New York Times.