Yes we're talking about the S&P500 but I'd like to point out that this isn't a general rule of markets; past performance does not guarantee future results. The S&P500 was at around 60 in 1960 and near 2000 today. It doesn't matter when you bought or at what intervals, money invested has performed well.
Still works out great.
http://awealthofcommonsense.com/worlds-worst-market-timer/