The tech-heavy index crossed over the 9000 mark on Thursday for the first time ever. Photo: brendan mcdermid/Reuters

The Nasdaq Composite Index took a detour on the way to its latest milestone.

The tech-heavy index crossed over the 9000 mark on Thursday for the first time ever. It pierced the level in early trading and closed up 0.8% at 9022.

The 9000 mark, however, took the Nasdaq longer to reach than other recent milestones. It crossed over the 8000 mark on Aug. 27, 2018—or 335 trading days ago, according to Dow Jones Market Data.

The index took 164 days to go from 7000 to 8000, between January and August 2018, and 174 days to go from 6000 to 7000, between April 2017 and January 2018.

The delay between 8000 and 9000 was mainly caused by last year’s market swoon. Right after hitting 8000, the Nasdaq began falling with other major U.S. indexes. On Dec. 24, 2018, the Nasdaq bottomed out at 6193—a drop of nearly 23% from the August highs. The index didn’t regain the 8000 level until April of this year.

Going from that point to Thursday represents 176 trading days, in line with the earlier 1,000-point moves.

Fueling this run has been, as usual, tech stocks. The Nasdaq Composite comprises about 2,700 companies, which are weighted by market capitalization.

Tech companies have an outsize weighting—currently about 48%.

Nearly one-third of the entire index’s weighting comes from just five companies: Microsoft Corp. , with an 8.8% weighting, Apple Inc. (8.4%), Amazon.com Inc. (7.1%), Facebook Inc. (3.55%), and Alphabet Inc. ’s Class C stock (3.5%). Alphabet’s Class A shares, incidentally, are sixth (3%).

Not surprisingly, those five stocks are all having great years. Microsoft is up 56%, Apple is up 83%, Facebook is up 58% and Alphabet’s Class C shares are up 31%. Amazon is up 23%, lifted by Thursday’s 4% gain.

Still, if this move between 1,000-point plateaus seems long, it is shorter than some of the index’s other milestones.

On March 9, 2000, at the height of the dot-com boom, the Nasdaq crossed the 5000 level for the first time. It peaked a week later at 5049—and spent the next 15 years trying to get back to that level.

It didn’t cross 6000 for two more years after that, in 2017—4,308 trading days since first hitting 5000.

The index’s longest span between 1000-point milestones came early. On July 17, 1995, the Nasdaq crossed 1000 for the first time. It took 6,171 trading days to get there from the index’s first trading day on Feb. 5, 1971.

Write to Paul Vigna at paul.vigna@wsj.com

Corrections & Amplifications
The Nasdaq Composite Index took 174 days to go from 6000 to 7000, between April 2017 and January 2018. An earlier version incorrectly said it took 174 days to go from 6000 to 9000. (Dec. 26, 2019)

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