Today, let’s ignore journalistic convention and discuss something before it happens rather than waiting until afterward. What I’m talking about is the Dow hitting 20,000, something people have been awaiting for about a month now. And that would have happened on Friday had any of the 30 stocks been just 5 cents higher when the average hit 19,999.63 in afternoon trading.

We journalist types generally don’t write about the Dow hitting some landmark number until the Dow ends the day above it. For ­example, even though the Dow briefly hit five digits — a major landmark — on March 16, 1999, that wasn’t declared 10K Day ­because the Dow ended the day in four-digit land. The Dow didn’t close above 10,000 until March 29, by which time the celebration felt as flat as champagne from a bottle that’s been open for almost two weeks.

So I’m writing about Dow 20K now before we all get buried in the endless articles, columns, blogs and whatnot that will be written when the Dow closes above that round but meaningless number.

[The Dow’s climb to 20,000? Whoop de doo.]

Rather than discussing yet again why the Dow Jones industrial average — to give the Dow its full name — has virtually no ­financial significance while the far-lower-profile Standard & Poor’s 500-stock index is the one that really matters, let’s have some fun with numbers.

Let’s start with the perception — which I had shared until recently — that the performances of the Dow and the S&P don’t diverge all that much. I hear that a lot when I write that the Dow, a 120-year-old average based on stock prices, has only psychological importance and that the indicator that really matters is the S&P 500, which is a 60-year-old index based on companies’ market values.

Lots of us, including me, own S&P index funds, so we have a rooting interest in how it performs. Virtually no one has a Dow index fund, although the Dow has vastly greater public mindshare than the S&P does.

In fact, the performance of the Dow and S&P aren’t all that far apart — but they’re far enough apart to make a difference, especially over a lengthy period.

According to the folks at S&P-Dow Jones Indices, which was formed in 2012 when the once-bitter rivals at S&P and Dow Jones combined, the Dow has ­risen by 6.3 percent a year, compounded, since the S&P assumed its current form in early 1957. The S&P has risen by 6.7 percent.

We’re talking price only. We can’t compare the indicators’ total returns — price gains or losses plus reinvested dividends — because the Dow, launched in 1896 before computers existed, isn’t set up to handle total return calculations. That’s a major reason the S&P rather than the Dow is considered a key performance benchmark by professional investors and has more than $2 trillion indexed to it, with almost nothing indexed to the Dow.

Those return numbers — 6.7 percent and 6.3 percent — don’t sound all that far apart, do they? But over time, the difference has been substantial. A hundred bucks set aside at year-end 1956 that earned the S&P rate would have been worth $4,797 at year-end 2016, compared with only $3,957 for the Dow.

Even though the Dow substantially outperformed the S&P last year, rising 13.4 percent compared with 9.5 percent, the S&P has outperformed the Dow in 33 of the 60 years covered by the numbers S&P-Dow Jones sent me. In some years, the difference was even larger than last year’s. For instance, in 1979 and 1980, the S&P rose a total of 38.1 percent, compared with 19.1 percent for the Dow. The Dow way outperformed the S&P from 1999 to 2002, with its four-year returns totaling minus 4.9 percent, compared with a minus 27 percent ­total for the S&P.

A brief aside — I used to have a lot of fun getting the folks at S&P to badmouth the Dow, and the folks at Dow to badmouth the S&P. It’s the old journalistic game. But after Rupert Murdoch’s News Corp., which took over Dow Jones in 2007, sold 90 percent of DJ’s averages-indices business to the Chicago Mercantile Exchange’s parent for $607.5 million in 2010, the S&P-Dow Jones rivalry seemed to get less intense. And now, of course, the S&P and Dow Industrials have the same owner.

In 2013, Dow Jones sold the balance of its averages-indices business to the CME for $80 million. This means that Dow Jones no longer has any financial stake in the Dow average, although two people from Dow Jones’s Wall Street Journal sit on the committee that picks Dow components.

Whether stock prices rise or fall matters a lot, especially to those of us — including me — who have a substantial investment in stocks. But when the Dow happens to hit a round number like 20,000 is a fluke. The Dow would have long since exceeded 20K had it not kicked out AT&T in 2015 to make room for Apple. AT&T’s stock price has risen since then, and Apple’s has dropped.

So maybe the Dow will close above 20,000 one day this week, or maybe it won’t. Barring a big decline, we’ll see 20K Day sooner or later. Will Dow 20K matter? Not in the slightest. But it will sure be fun to watch the festivities.