Los Angeles Dodgers starting pitcher Clayton Kershaw smiles during a new conference Thursday at Nationals Park. (Alex Brandon/AP)

This is how the world thinks of Clayton Kershaw from April to September.

“He’s the best pitcher on the planet, if you want to call him that,” Nationals first baseman Ryan Zimmerman said. “If he can do what he has done for another three to five years, you’re talking about — nobody ever says it — but maybe the best pitcher of all time.”

Just checking the history books here. Turns out “all time” spans a lot of years.

“Nobody ever throws that stuff around,” Zimmerman said. “But his numbers are a joke.”

Well, that’s if you think a 2.37 ERA over the course of a career — a career that’s still in midstream — is a joke. The number of starting pitchers whose careers have begun within the last century who have a better ERA than that: zero.

The Post's Jorge Castillo and Chelsea Janes preview the NLDS between the Nationals and Dodgers. (Jayne Orenstein/The Washington Post)

Yeah, yeah. But that doesn’t take into account the era in which he plays, and Kershaw plays in an era in which hitters strike out more frequently than at any point in history — the ninth consecutive season that rate (now up to 21.1 percent) has set a record. Since the game ostensibly cleaned itself up (at least a bit) with drug testing, it has tilted toward pitching.

Take, then, adjusted ERA+, a statistic that considers a pitcher’s ballpark and the league norms for the time he pitched. The number of starting pitchers in history who have a better number than Kershaw: zero. Better than Walter Johnson. Better than Pedro Martinez. Better than Roger Clemens.

But Friday, the calendar says it is October. And slice it any way you want, but in October, Kershaw has been — how to put this? — ordinary.

That might be kind.

“I don’t know how many starts I’ve had in the regular season but hundreds,” Kershaw said Friday afternoon at Nationals Park. “And you don’t have that opportunity in the postseason. So you’ve got to make it count, and the bad ones stand out more, for sure.”

A little exercise in statistical comparison: Kershaw’s ERA in 13 postseason appearances is 4.59, more than two runs higher than the regular season. Eleven times in his 21 outings during the 2016 season, Kershaw didn’t issue a single walk. In those 13 postseason appearances, he has walked at least one batter 12 times. In 265 regular season appearances, he has allowed as many as seven earned runs four times. In those 13 postseason outings, he has given up that many twice.

So who, exactly, will the Nationals face in Friday’s Game 1 — a candidate to be the best pitcher of all time or a guy who sends jitters through his own fan base because of uneven October performances past?

“I don’t read too much into it,” Dodgers Manager Dave Roberts said. “I haven’t looked back on it. I don’t think it has any bearing on this season, this postseason, the start tomorrow, and I really don’t think Clayton cares either. All things are one pitch here, one pitch there. Like I said, [there’s nobody] I’d rather have on the mound tomorrow than Clayton Kershaw.”

In understanding Kershaw’s place in regular season history, it’s important to note that — at just 28, a veteran of nine major league seasons — it’s possible he has as much baseball in front of him as he does behind. Even if he continues his excellence, maintaining a 2.37 ERA into his mid-30s seems unlikely.

But in understanding his postseason history, it’s important to consider the intricacies of what has happened to him. He has, in all honesty, just three truly shaky playoff starts. They overshadow, for example, the seven innings of one-run ball he tossed on three days’ rest in last year’s division series against the Mets, forcing a fifth game. Still, they should be parsed.

In Game 1 of the 2009 division series against the Phillies, Kershaw had allowed just one hit through four innings. But he opened the fifth with a single, a wild pitch, a walk to set up a force play — and a three-run homer from Carlos Ruiz. With two outs, Ryan Howard drove in two more with a double, and Kershaw was lifted.

That came relatively early in his career, before he won any of his three Cy Young Awards. The other two starts that got away from him seem fresher — and worse.

In 2013, he had allowed one earned run in 19 postseason innings over three starts when he took the mound for Game 6 of the National League Championship Series against the Cardinals. With a run in and two out in the third, he allowed five consecutive base runners. He then opened the fifth by allowing two singles and Matt Adams’s run-scoring double, came out of the game trailing 5-0 and watched the bullpen allow the two runners he left on base to score. In the books, that goes down as seven earned runs. The Dodgers lost, 9-0, ending their season.

And in 2014, in the first game of the division series against St. Louis, he was pitching an absolute gem. Yes, he allowed a pair of solo homers. But in between, he had retired 16 straight men, walked none, struck out eight. The Dodgers led 6-2.

Yet Kershaw didn’t survive the seventh. On his final pitch, Matt Carpenter drilled a three-run double that gave the Cardinals the lead. He was charged with eight runs.

“I feel terrible,” he said afterward.

It is the nature of evaluating the best practitioners of a specific craft that their failures must be examined, and so those games have come to represent October Kershaw. But he said Friday that this odd season, in which he missed two months with a back issue the Dodgers feared might sideline him for the rest of the year, has allowed him to evaluate his place on the team a bit differently as he begins the playoffs.

“I think in the past, I’ve definitely felt that pressure more,” Kershaw said. “But this year’s been a little bit different for me, just as far as having to watch on the sidelines for two months. . . . I think it’s really kind of hit home for me a little bit as I’ve come back that I can definitely be a part of this and definitely help and definitely be a factor in winning. But I don’t have to be the factor.”

It is a star’s predicament that he is evaluated against his own excellence. Kershaw’s excellence is historic, bordering on best of all time. Friday presents another opportunity for his October self to match his regular self. If it does, the Nationals are in trouble.

Which Clayton Kershaw will the Nationals face in Game 1 Friday?