The extension sets every DuckDuckGo search to your chosen date range - past day, past year, etc. The chosen range will continue working until you choose "None" from the dropdown.
Custom date range works, but it's difficult to get to. First choose "custom" from the dropdown, click Save, then click out of the extension and then back in. Now you should see the calendar date picker to choose dates. I am working on fixing this.
Want to change this? Source code is here:
https://github.com/aglines/search_ddg_customdaterange
Source code:
https://github.com/aglines/search_ddg_customdaterange