This question is in reference to Atlassian Documentation: Adding the Calendar Gadget
How can I add issue calendar gadget to a Conflunce page?
You linked to JIRA issue Calendar.
To display issue due dates in Confluence, AFAIK you cannot use JIRA Issue Calendar at all - you'll need Confluence Calendar or Confluence Team Calendars.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.