well calling $this->seminardate before the object has written could be the issue as that function is run 'before' you save your object, so $this->seminardate may not exist. You might have to use onAfterWrite to update it instead
It leads to no js error but does not write at all
ah well of course im guessing issue there is of course you can calling this function 'after' the write - the write saves it to the database, so can you see the problem :P you need to call write() so it updates it in the afterWrite - but that calls afterWrite which calls write again and we loop on and on!
Looking at some code, onBeforeWrite() might be what you want and I think the error was you are not calling parent::onBeforeWrite(); in that function at the end. Try that and see what happens
you can get the date from the dataobject which you create and add 7 days to it, and try to work with time epochs it will be the best,
Thanks! I have seen it. It is too complicated for me so far. But at some moment I will look either diary or event_calendar and probably both