I have tried it in the latest version of 4.3 that we are working on and it works in the following way:
1. add SetVariable activity with two DateTime variables and set Prompt checkbox - For example date_start and date_end
2. make the variable values to be surrounded by single quotes - For example '2015-05-01'
3. add your desired database activity that is supposed to get data (Execute script activity maybe or Select to file?)
4. use variables in your SQL query and surround them by single quotes - For example SELECT * FROM myContacts WHERE brith_date between '#date_start#' and '#date_end#' (please note that single quote indicating datetime string literal and the hash sign indicating variable identifier)
However you will have to try this in the next comming beta, because i was not able to achieve this in 4.2 due to the above mentioned datetime bug.
Hope this helps