You have to decide what you want to happen with the records where the field = "Product Summary", but the date is null. Maybe you want to set the date to today's date or maybe you want to ignore them--only you can tell us.
If you want to ignore them, you can set up a formula for the date where nulls are set to some future date, as in the following:
//{@date}:
if isnull({table.date}) or
trim({table.date}) = "" then
"2999/09/09" else //set this up in the same format as your field displays
{table.date}
Then use this formula:
if {table.field} = "Product Summary" then {@date}
Then you can insert a minimum on this.
-LB