I do know that if you delete a post it does not change the last post date, so it requires a new post after that before it gets back in sync.
Other than that I'm not able to produce the problem you describe, maybe you could try to produce it on demo.mojoportalcom so I can see it. Or perhaps if you tell me more about configuration settings you are using like if you are forcing a specific culture or any other factors that may be involved. You did not mention your version of mojoPortal.
Note that the time zone system in mojoPortal has changed over time. You may need to make sure that you have the correct time zone set on your user profile. You should also set a default time zone in Site Settings because that is used for unauthenticated users.