I would love to see a user customisable setting (via preferences) that lets the user select the date range viewable in the Sales and Purchases Register, it'd also be productive to have default buttons that show "THIS MONTH", "LAST MONTH" so that you can see at a glance your register for the chosen period.
In preferences I could set up SALES REGISTER "Custom Display" to be 7 days
In preferences I could set up PURCHASES REGISTER "Custom Display to be 14 days
When I'd go into SALES REGISTER (or purchases register), I would see the date range as normal, however there'd be click choices to populate these fields with either "CURRENT MONTH", "LAST MONTH" or "CUSTOM DATES" - it's painful having to type in them, especially for the week or so after a change of month. (You'd still need to be able to enter the date range you want manually, but the overall "defaults" would streamline everything.