FileMaker 6 fields in SQL Query

I hope someone can help!

I have the following SQL Query (which works fine)

select * from invoices

where inv_date >= '01 Jun 08' and inv_date <= '30 jun 08'

However I want to parameterise the two dates with global date fields stored in FileMaker 6, in a database called 'dates' (saves manually changing dates at start of every month). The query would then look something like:

select * from invoices

where inv_date >= '&&start_date_global' and inv_date <= '&&end_date_global'

This is where I get stuck! How do I make the FileMaker 6 database 'dates' (and the two fields 'start_date_global' and 'end_date_global') be visible when building the SQL Query. And then make them work in the query.

Thanks in advance!

Duncan