The study uses daily closes of the S&P 500 index from Jan 3, 1950 to Oct 23, 2026, price only, without dividends. A record close is a close above every earlier close in the series. Study days start in 1955, because the index first passed its 1929 peak in 1954 and the series begins in 1950. That gives 1,327 record days out of 18,074 trading days.
Forward returns run 21, 252, 756 and 1,260 trading days (about 1 month and 1, 3 and 5 years), close to close. Recent days without a full window are left out. Windows overlap, so the days are not independent and the sample is smaller than the counts suggest. Dividends would add to every figure, for records and all days alike. These are past results of one index. They describe how the market behaved, not how it will behave.