One of the features of SharePoint 2010 Enterprise is the ability to provide a social networking experience through My sites. The My sites features an user profile that allows the user to customize the My site time zone along with the activities they desire to get feeds for. Some of these feed events, like Anniversary Feeds, may show up before or after the expected time, depending on which time zone the server that generated that feed is in.
Feed generation happens via timer job, scheduled for running in fixed intervals. In a SharePoint farm environment, any server can potentially execute this timer job independently of where the server is located. For many reasons, UTC is used as time unit.
When executing the job, the determination to generate a feed is done taking in consideration the date and time in UTC format. Depending on which time zone users set on their profile, the feed event may appear either ahead or behind their local time.
Take for example a user with their profile set to Central Standard time and having a birthday on August 15. Central Standard Time is UTC-6, meaning that this feed could be generated at 6:00 PM and would show up on the recent activities before midnight of the user's birthday.
This behavior is expected and in accordance with the product specification.
My Site, Feed, Social Networking, birthday activities, SPS2010