Generated ghost events should start generating as early as the range start minus the duration of the event.

Summary: Fixes T8347, Generated ghost events should start generating as early as the range start minus the duration of the event.

Test Plan: Create a year long weekly event, a year later, each day should have about 52 events.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8347

Differential Revision: https://secure.phabricator.com/D13069