A self join could be the simplest possibility here to set up.
However, you appear to have one date field that applies to a group of records, that suggests you might set up a table of related records with one record in this table for each such group of records. You could put your date field in this table and all your records in your current table could link to it.
Of course, it is frequently said, "this forum rocks". Phil was the only respondent and quickly pointed me in the right direction.
Thank you so very much. It took me a couple more hours, including the following documentation, which I'm happy to share in case someone else needs the help.
/Users/dstark/Dropbox/Conditional Formatting Example.jpg