The Group Filter widget doesn't seem to have the "Between" operator like the regular Filter widget has. So, to filter by a date range I believe I need to choose "GREATER THAN OR EQUAL TO", then click the add criteria button to select "AND", and then select LESS THAN OR EQUAL TO.
My users will always be doing a date range search, so I want it to display the following when the Group Filter widget opens (rather than have to go through the process of selecting the operators).
I modified the text in the widgets\GroupFilter\nls\String.js. Thank you.