In the wait for Dow 20,000, the last mile feels the longest

The climb toward 20,000 on the Dow Jones industrial average may feel more like a crawl, but the index's movement isn't unprecedented.

The Dow came within 0.25% of 20,000 for the first time back on Dec. 13 – just more than two weeks ago. Investors have waited nine sessions to hit the milestone since then, and it'll be 10 if the Dow fails to rally hard in late Wednesday trading.

But the Dow waited 18 sessions after coming within 0.25% of 17,000 before it actually hit that milestone intraday, and it waited 12 sessions after coming within 0.25% of 18,000 before it hit that intraday.

Still, except for those two instances, the Dow has historically hit the milestone soon after coming within 0.25% of a milestone.