Today is NBC Day. It's the day on which the A.C. Nielsen Co. will make official something that's been blatantly evident for weeks: that the NBC Television Network has won its first prime-time season ever.

This isn't just good news for NBC. It's good news for ABC and CBS as well. It's good news for television, because NBC has not only won the season but done much to restore public faith in network TV. NBC has helped make prime time safe for civilized human beings again.

In addition, the other two networks have already begun to follow NBC's lead in the way networks deal with producers and shows, giving producers creative independence and shows a more generous opportunity to build audiences before being yanked. NBC has revolutionized the creative environment in television. The effects may be seen on the other two networks already.

The Nielsen-rated TV season that ended last week was one in which the continuing erosion of network audiences -- as viewers drift off to alternative forms of television -- was, at least momentarily, halted. A gratifying, positivist, impeccable hit like "The Cosby Show" does more than bring people back to network TV by the millions. It helps restore lost honor.

NBC has the reputation of being not only hugely successful but honorable, at least within the Darwinian context of network competitive savagery. The man most responsible for the prime-time triumph is probably bouncing Brandon Tartikoff, president of NBC Entertainment. But the man who has changed NBC's image from lowly loser to lustrous leader is chairman and chief executive officer Grant A. Tinker.

"I'm taking it very quietly," Tinker said yesterday from his office in Burbank, Calif. "Brandon of course is stoned out of his bird. No, he's calm, too." The champagne had not yet been broken out to celebrate the unprecedented ratings victory, Tinker said. That was to come later at a celebratory party held in a nearby hotel.

Among the invited guests was Sylvester L. (Pat) Weaver, who presided over NBC during its first and, until Tinker, last real heyday -- the 1950s, when Weaver led the way in programming innovations like the "Today" and "Tonight" shows and NBC, under parent company RCA, led the way in introducing color television.

Tinker and Weaver are different generations of the same breed of imaginative, socially responsible network executive. Last week, at its annual convention, the National Association of Broadcasters gave Tinker its highest honor, the Distinguished Service Award. He's about as well liked as a network boss can get.

Jay Sandrich, director of "The Cosby Show" and of "The Mary Tyler Moore Show" in the days when Tinker ran MTM Enterprises, says, "I don't know anyone who has ever worked for Grant Tinker who wouldn't walk off a cliff for him."

Not wanting to sound uncustomarily elated about the Nielsen win, Tinker insisted on taking it in stride. "I am getting some feeling of satisfaction out of it," he conceded. Then, worriedly, he said, "Now, if we can just make a habit out of it."

His major satisfaction, Tinker says, comes not from the prime-time win but from "some very important quiet victories in other areas," citing NBC's pioneering use of Ku-band satellite delivery of programming to affiliates and the growth of stereo TV, an area in which "we're light years ahead of the other guys."

A class act all the way, Tinker also paused to rave yesterday about Sunday's CBS broadcast of the Vladimir Horowitz recital from Moscow. He said it was "wonderful."

At NBC, these are almost spookily prosperous times -- surely the good old days of the future. ABC is suffering under the cost-cutting and layoff binges of new owner Capital Cities Communications, and finished the season a dismal third. CBS is still smarting from costs of fighting hostile takeover attempts and is now humiliated by having the prime-time crown snatched away.

Meanwhile, NBC's "Today" show has overtaken long-time leader "Good Morning America" for top spot in the morning ratings. NBC leads in late night with venerable Johnny Carson and irrepressible David Letterman. Even the daytime schedule, traditionally NBC's weak link, is improving in ratings.

"The only thing left, really, is the 'Nightly News,' " Tinker said. Tom Brokaw recently tied traditional leader Dan Rather in the ratings. Some NBC News insiders privately predict Brokaw will manage the heretofore unthinkable and actually overtake Rather before the end of the year. "I think we can expect that," Tinker said yesterday. "We're sort of cocky about it. I think that will be the thing that will shake up the establishment more than anything else we've done, especially along -- what is it you call it? -- the Eastern Corridor. That will be kind of fun."

All is not well at NBC News, however. The division's attempts to produce a competitive and respectable weekly news magazine have so far verged on the farcical. Tinker has now seen yet another revised version of the "Almanac" show with Roger Mudd and Connie Chung. One NBC insider who's also seen it yesterday bemoaned it as "awful." Tinker called it "a step forward" from the last try, which died after a short life, and said that what he told NBC News President Lawrence K. Grossman about the show was "largely positive" and constituted a go-ahead.

"I would guess he is not far from a firm air date -- as early as June. He certainly has my blessing," Tinker said. An NBC spokesman said the magazine will be on the new fall schedule that NBC will announce May 15. But the old boastful commitment to the show -- that it will stay on no matter how poorly it does in the ratings -- has now been qualified. The spokesman said the program will stay on only if it is of "quality" and "worthwhile" and "what we had in mind when we first put it on."

Asked if the decision to go ahead with the magazine was the right one, Tinker said, "Who knows whether it's right. The audience makes that decision."

Hampering the NBC euphoria is the fact that Tinker has indicated he will leave the company once the takeover of NBC parent RCA by General Electric is complete -- probably in August or September. Insisting that NBC's turnaround was "a team effort" and that he is not irreplaceable, Tinker says he let the news of his plans to leave slip out gradually in hopes of "getting people in the building used to the idea." Last December he was already toying with possible logos for the new production company he wants to start.

"I am absolutely sure the place is just going to be humming when I leave," he said yesterday. "I know the people who are going to take up the slack. I work with them every day. They are doing the work, and I'm just taking the bows. If I really thought the company was in any danger, I'd bite the bullet and move to Secaucus and commute, just to stay on. But I don't."

If Tinker goes, broadcasting insiders hope his influence will remain. He has brought a new civility to the network television business, and his way of dealing with producers has made him a Hollywood hero with no equal. Network competitiveness is as breakneck as ever, but Tinker has humanized it.

One could argue that NBC's first-ever winning season was also the first season of something called Tinkervision, which is a rarefied form of television in which quality is given equal status with ratings and both the audience and the program makers are treated with respect. If it's going to get even better, it could end up being great.