Problem calendar if timezone is empty

Hi, approximately 30,000 appointments are stored in the Horde database, roughly half of which do not have a time zone stored. That works fine in itself, but lately it has been causing problems in Thunderbird calendar (CALDAV). If I have analyzed it correctly, this is because Thunderbird's calendar converts the null value to UTC in its cache (sqlite databases). That is not the correct value. So registering an event works fine, but when you reopen Thunderbird, the display is incorrect. A solution could be that the cache sqlite database treates timezone differently. I understand from others that time zone is not a mandatory field. It can therefore be left blank. I suspect that Thunderbird calendar cache is making a mistake here.