Pace connects to your iCloud calendars over CalDAV, two-way
Connect iCloud with an app-specific password and Pace reads and writes your Apple calendars over CalDAV: the same calendars you see on iPhone, iPad and Mac.
Apple has no sign-in button for calendars, so you generate an app-specific password at account.apple.com and paste it in. Takes about a minute. Same calendars you see on iPhone, iPad and Mac.
Pace writes the plan's sessions into the first calendar on your iCloud account (or into Google, if you connected that first), and sees when you move or delete one.
If it shows in your iCloud calendar list, Pace can read it, and you choose which ones count.