I found it somewhat difficult to set long date ranges on notes using iOS, so I made this shortcut.
From the note that you want to use, share the Agenda Link to this shortcut. It then asks you for the start date (type in a date using natural language: e.g., “May 1 2020”) then the end date. Note is then updated.
https://www.icloud.com/shortcuts/a0ac89bcc8b6460399193e8c3b41eaa7