Hi Steve,
That's 610 trading days from the major secondary top on 9-1-2000. It's a Fibonacci turn date; the next one off that date won't be until day 987, or next year some time.
The last Fibo turn date off 2000 was September 3, a 90% downside day that came 610 days off the S&P's ATH. Not sure if tomorrow will be as dramatic; the Dow's 610-day turn off its ATH was a dud (June 18, I believe).
Before that, I think the 377-day turns came in around Sept. 21, 2001, but I forget exactly when.
Just something I track for potential turns; Feb. 19-21 has a good "cluster."
Cheers,
Paul