This error will show up only, if I'm not logged in. Otherwise it shows "15:21" for my "Time difference" posting. (Depending on my "Time difference" user setting.)

I don't really understand what you mean. How many days/hours/minutes ago a message was posted is completely independent from the time zone.

To be honest, I never fully understood the timezone stuff.

Here is an explanation.

The time difference option in the user settings is there in order that you can see when a message was posted at your local time if the forum time is different to your local time.

Alex