If this were Stephen Sondheim’s heyday — by which I mean the 25-year period, from 1970 to 1995, when this songwriter was the dominant creative force in the American theater — he would surely be starting work on a musical about the coronavirus.

Sondheim never hit box-office gold, which isn’t surprising when you consider the coronavirus-esque material he wrote about — like “Assassins,” which is about what it sounds like, and “Sweeney Todd,” the operatic account of a Victorian serial killer.

And who else but Sondheim — who celebrates his 90th birthday Sunday — would have thought the opening of Japan to Western trade in the 1850s would be fertile ground for a musical?

Alas, the people who backed his somewhat obscure show “Pacific Overtures” in 1976 probably ­regretted being talked into the idea, since it only ran for 193 performances on Broadway and lost its entire $650,000 investment (about $3 million today).

But it features a Sondheim score so rich, so dazzling, so witty, so variegated and so emotionally resonant that even after hundreds of listens, its original-cast album can still bring tears to my eyes.

I think especially of “Someone in a Tree.” In this seven-minute number, Sondheim recreates a dry historical event — the treaty negotiations between Japan and the United States — through the eyes of an old man who many decades earlier watched the proceedings silently from a tree branch and his 10-year-old watching self.

“Someone in a Tree” is a profound meditation on youth, age, understanding and history itself. The watcher is, the song says, a fragment of the day, the ripple and not the stream — but if he hadn’t been there, who’s to say what happened would have happened?

It is one of the greatest songs ever written. You’ve likely never heard of it. And I’m not even sure it’s in the top 15 among Sondheim numbers.

There are the ones to which he wrote only the lyrics while he was in his 20s, like “Somewhere” from “West Side Story” and “Mama’s Turn,” the ultimate closing number of the best of all Golden Age musicals, “Gypsy.”

There are the absurdly clever pastiche songs he wrote entirely by himself, in which he repurposed classic forms of the popular song to astonishing effect, like “You Could Drive a Person Crazy” from “Company” and “Broadway Baby” from “Follies.”

There are the songs of heartfelt, even tragic yearning, like “Not a Day Goes By” from “Merrily We Roll Along.”

And there are the rueful, hard-won-wisdom songs, in which older, sadder and wiser characters take responsibility for their mistakes — like his most famous, “Send in the Clowns” from “A Little Night Music.”

Sondheim is a savage ironist, so perhaps the greatest of all his savage ironies is that he wrote these peerless tunes for shows that were, more often than not, emotionally unsatisfying.

The dominant feeling expressed through his most personal shows, from his breakthrough “Company” in 1970 through 1994’s “Passion,” is deep ambivalence — about the durability of any kind of relationship, about the demands for compromise that are the hallmark of any adult enterprise and about humanity itself.

And yet time changes things. I saw “Sweeney Todd” in its original run on Broadway more than four decades ago. The criticism at the time was that it was too punishing, too dark. Sweeney Todd slashes throats in full view of the audience and sings a love song to the razors he uses.

If you had told me that, four decades on, my own 15-year-old daughter would perform in this show at her high school, I would have said you were insane. And yet she and her schoolmates did it.

And every second of it was ­astonishing, even revivifying. That is what great art — and “Sweeney Todd” is perhaps the greatest single work of the American stage — does.

And Stephen Sondheim, New York City’s greatest living artist, has given us songs that will be sung and laughed with and cried over until people stop singing. Which could be next Tuesday.