Even though the great recession was ongoing for almost a year prior to being dated by the NBER, it was the announcement itself that started the massive migration from stocks to bonds. The purple square encapsulates this time range. It is best to enter this position before an announcement, which some market participants did.
At the current growth rate, we will be in a recession anywhere around October - January, after mass layoffs result in unemployment across the economy. The NBER has responded more quickly in dating the more recent COVID-19 2020 recession, which lasted only a few months and announced around four months after it started.