The 20th century ended on May 14. A world without Frank Sinatra is indeed an emptier place — a gap exists in pop culture that no artist can fill the way he did.

Sinatra is the only celebrity marker who extends from World War II into the early ’90s, defining time and eras as though he alone were authoring them. As a singer especially, Sinatra dodged his own past when he needed to and embraced it when he was ready. He approached music like a boxer facing a tough opponent, ready to bob and weave or lurch forward to meet the next challenge head on, the challenge most often being the material — a standard or a new work from one of the songwriters he put on the map, such as Jule Styne, Sammy Cahn or Jimmy Van Heusen. He never lacked for confidence and one has to wonder if ever had a moment of self-doubt.

In 1943, he arrived on 78 rpm discs when Americans needed a hero; women, stricken with the pain of a loved ones away at war, Sinatra’s voice was an emotional salve that made a beeline to their sunken hearts. For all Americans, especially those from Italy, he was the ultimate immigrant hero, the one who assimilated and created something decidedly red, white and blue.

As he aged, fans grew with him and legions more signed up to be “in the moment” just as Sinatra always was. In his mid-’50s heyday, arrangers such as Nelson Riddle framed and supported him with the tempo of a heartbeat, establishing a visceral connection with lovers young, reunited and hip. His ’60s days were about swinging and good times. Only in the 1970s did his material veer toward the uninspired or too hinged to the past.

His ’70s and ’80s concerts, however, more than made up for it — Sinatra was laying the groundwork for the acceptance of the Great American Songbook in the 1990s while solidifying his own canon and image.

A Daily Variety reviewer wrote after the first of an 11-night, sold-out run at the Universal Amphitheatre in 1978: “Sinatra has refined his personal style to the point where every gesture and vocal inflection becomes part of the legend, elevated beyond the significance of the moment.”

Yes, what he did on the concert stage was always of vital concern — Daily Variety saw fit to review 34 of his performances between March 1976 and September 1992. As for television, Sinatra was one of the last voices to bring, well-performed classic American music into burgeoning rock ‘n’ roll households. He sang it straight, he sang it casual, he sang it with conviction.

“I don’t know what other singers feel when they articulate lyrics,” Sinatra told Playboy magazine in 1962 in an interview that snared Frank’s feelings about disarmament, communism and religion. “But being an 18-karat manic-depressive and having lived a life of violent emotional contradictions, I have an overacute capacity for sadness as well as elation. … I’ve been there and back. I guess the audience feels it along with me. They can’t help it. Sentimentality, after all, is an emotion common to all humanity.”

His best recordings seemingly suspend time, capturing the crispness of a fall day in New England with an ease that seems as natural as breathing to Sinatra yet impossible for any other mortal to replicate. And like an athlete told to hit the showers before his time is up, Sinatra returned time and again from being “washed up” to hit yet another peak.

George Siravo spent much of the 1940s arranging Sinatra’s swing sessions and put together a touring songbook for Sinatra before he signed with Capitol. In the liner notes of “The Best of the Columbia Years” four-CD set, author and critic Will Friedwald, who writes with unparalleled authority in interpreting Sinatra’s vocal abilities, quotes Siravo: “Frank is to be credited with making a lot of arrangements sound better than they would have with somebody else singing them because he had the expertise and the innate ability. He’s sly, he’s smart, he’s cognizant, he’s always intelligent, and of course he knows how to use a beat. I can’t think of one singer that didn’t learn from Frank.”

And that would have to include Frank himself who developed from a light-baritone crooner to a swinging saloon singer and full baritone to a worldly voice with an uncanny touch singing words of introspection. Some would say you could discount the first dozen years of solo recordings and head straight for 1955 and the five sessions that became “In the Wee Small Hours,” the disc that started Sinatra alternating between swinging machine and saloon singer. (It’s also his best disc.)

But in those early years up through Sinatra’s beginnings at Capitol, the arranger and conductor Axel Stordahl allowed the phrasing to stand front and center over a beatless wash that allowed Sinatra to directly address his audience with heartfelt sentiment. At times it was overdone, which surprisingly never affected sales, but his framework would regularly crop up in Sinatra’s later work.

Sinatra was 37 when he started Phase Two of his career and created the indelible image people see when they close their eyes and think of Frank: Capitol Records. Nelson Riddle, Gordon Jenkins and Billy May arranging. “I’ve Got the World on a String” kicking it off in 1953. Grammy awards in 1959 for best vocal and best album for “Come Dance With Me.” The foundation of the Rat Pack. Joey Evans with a drink and a song in “Pal Joey.”

He became the hip and carefree swinger with the 1955 album “Songs for Swingin’ Lovers” and followed it with “High Society” and “A Swingin’ Affair,” the Riddle arrangements setting the tempo at a steady heart beat. In the hands of Gordon Jenkins he began his “saloon songs” period, starting in spring 1957 with the album “Where are You?” and the ultimate classic “Frank Sinatra Sings Only for the Lonely.”

From 1956-58, he recorded with Riddle, Jenkins and May, cutting definitive versions of “Witchcraft,” “I Didn’t Know What Time it Was,” “The Lady is a Tramp,” “All the Way,” “Chicago,” “Night and Day,” “Come Fly With Me,” ” I Wish I Were in Love Again” and his signature saloon song — “One for My Baby.”

He moved on as rock ‘n’ roll and the Brits took over, establishing his Reprise label and stunning the 1966 hit parade with the No. 1 single “Strangers in the Night.” He used the decade musically to show he could swing with Count Basie’s band and generate the notion that he could indeed be a jazz singer, he turned to the Brazilian music of Antonio Carlos Jobim, to Broadway, to the Beatles, to New York and eventually to his admirers that made his two “Duets” albums his best out-of-the-blocks sellers.

The Voice. As recognizable as that of your mother and sometimes just as soothing. Even when it was slightly off, it was easy to forgive because of the history implied in each breath and the unfettered hope that the next line would hit you with a wallop bigger than the last.

So put an X on the date May 15 because this will be known as “The Day After,” the one author David Hadju warned us about in his book “Discovering Great Singers of Classic Pop”: “To hell with the calendar,” he wrote. “The day Frank Sinatra dies, the 20th century is over.” Start the 21st today.