You may occasionally see guests that get a birthday or anniversary reward at an unexpected time(e.g. a campaign is configured to deliver the reward 7 days before the guest's birthday. In reality, guests are being rewarded the gift 2 or 3 days before their birthday, or at different times of the day).
Having an anniversary campaign schedule configured will correctly trigger which guests receive the reward and at what time. This will ensure that ALL eligible guests are captured and rewarded, appropriately.
If a schedule is not set, you'll see inconsistent(day/time) rewarding of these campaigns, which may look like it is not working. These can be configured by clicking Edit Schedule/Schedule under Campaigns > Anniversary or by finding the section under Support > Schedules > Additional Schedules.