Before I show those dates I want to mention another breadth indicator I look at which also measures oversold breadth. Worden Bros. T2114 and T2116 measure the % of stocks trading 1 and 2 standard deviations below their 40-day moving average. Both are in extreme territory at the current time. T2114 (1 standard deviation stretch) is reading almost 87% currently. Data is available back to 1986. Since then there have only been 6 other periods where similar levels were reached.
Below is a table comparing the CBI>=18 to a Worden Bros. T2114 > 85:
Both indicators are now at similar extremes only reached during some sharp selloffs that resulted in sharp rebounds.