Is there a way to merge and see all your projects on one calendar?
If your projects are in the same team, use the Team Calendar to see them all together: https://asana.com/guide/help/organizations/team-pages#gl-team-calendar
Another, different, approach is to multi-home tasks from multiple projects to a master calendar project, then that project’s calendar will show these tasks.
Hope those help,
Or use the advanced search and then look at the Calendar tab.