Strange, I have never seen that before.
Do you have more information available? Is the website based on an export and import, where the date went wrong? Did you alter any code?
Also, could you check the database? The table wp_gwolle_gb_entries should have a datetime field with timestamps as date. Are they all at 4 oktober in the database? If so, it is happening at creation time. If they are all different and correct, it is happening at display time.
And if you go to Guestbook > Settings > Debug-tab and test the adding of entries, are they listed at 4 oktober as well on the overview? How are the existing entries listed on the dashboard, do they have the correct date?
I can imagine there is a translation plugin that is mismatching the dates, could that be? Looking at network traffic in the browser it is quite a busy bee ??