Hi Nini,
Very strange. You might also try clearing your permalinks by visiting Settings > Permalinks – you don’t have to do anything else but visit that page and permalinks will be cleared. Another thing to try would be to change the time zone in Settings > General to either a UTC time or select a city.
Aside from that you’ll need to try deactivating all other plugins and/or switching to the default WordPress theme. Understandably you won’t want to do this on a live site so I would suggest setting up a development copy of the website to be able to test things out.
I hope that helps!