Post-market highs and lows show the highest and lowest prices since the market closed.  This only includes the post-market prints, which are not part of the normal highs and lows.

You can filter these alerts the same way as other highs and lows, with one difference.  We start counting the number of days from today's close.  So a value of 1 day means that the high was higher than today's high, but not higher than the previous day's high.  If that same print had happened before the market closed, it would have generated an alert with a value of 0 days.  More details