Hi, as you can see from my profile I have a lot of experience with Chrome Extensions.
Only way to make extension work with time period instead of day by day, is to make user go to [login to view URL] , download ZIP file with his location history, extract JSON file from it, and then open it within Chrome Extension.
Day by day is not a problem, we can add some menu with two buttons, something like this: "Add this day to CSV file" and "Finish and download CSV file". So user opens the days they want exported to CSV one by one and each time they click on "Add this day to CSV file" button, and when they are done, they click on "Finish and download CSV file" and get the finished CSV file.
If you have any questions, feel free to ask.
Regards, Stefan