upcoming_events shortcode  attributes
The upcoming_events shortcode provides some attributes. I want to use the selected_datetime attribute but the documentation doesn't show in what format to supply the date range and I can't get it to work. The documentation just says:
I've tried:
- [upcoming_events selected_categories="white-ribbon" selected_datetime="06/12/2025 : 06/12/2025"]
But it doesn't work.
Please can someone let me know how to use the selected_datetime attribute? 
Thanks.