Duplicate entries in subform combo box

[ccook241]

I've designed a database for biological data where the primary relationship is "Site." For every site, there is a set of dates in which we collected data. I have a combo box to look up records for a specific site, and would like to have a subsequent combo box to look up records from a specific date at that specific site. When I create the "Find a date" combo box, it includes the dates from ALL of the sites (there are triplicate entries for every date but only one date entry corresponds to the selected site). I'm looking for suggestions on how to create the "Look up date" combo box such that the listed dates are specific to the selected site. Please see the attached image for context. Any advice is much appreciated!

[pbaldy]

See if you can adapt this:

http://www.baldyweb.com/CascadingCombo.htm