Finally figured out how to successfully implement a data range search.

In plugins/vpn-search/form_advanced.php page, I added the two elements to the search form.

In plugins/vpn-search/vpn-search.php, I modified the getAdvancedConditons method (which constructs the conditions for the WHERE clause in the SQL query) by adding special cases to deal with table.field values of poems.po_date and poems.po_date2. There is no po_date2 field in the poems table, so the special case code inserts "po_date" instead of "po_date2". The reason I'm using a bogus table.field identifier is explained below

In plugins/vpn-search/classes/VPNFormBuilder.php, the fieldMap array uses a table.field name as the key and the id of the element in the GUI as the value. Code assumes a one-to-one relationship between GUI elements and table.field specifier. I needed to add two new key-value pairs, but the two new elements both are associated with the same table.field. I can't use the same key for more than one value, so I created a bogus table.field value (poems.po_date2) and then special-cased that value in the code in vpn-search.

At the moment, the date-range-start and date-range-end fields accept four digit years, and if crazy values are inserted, no checking is done and the user just gets 0 hits.