The magic string expression again got "corrupted" after I sent it. As you can see from other magic strings in your response, the last part is the default value which is used if a property doesn't exist, it has to start with /* and end with */.
So, the correct expression is {{=doDateDeltaFromCurrent("yyyy-MM-dd'T'HH:mm:ss", "0D");/*2018-10-30T00:00:00*/}}
(fingers crossed that it should work this time :-) )
I am starting to understand what happens, when my mail contains /*<someText>*/ it is converted by this site into /<someTextInBold>/ , so that's an attention point for any email posts in the future.
Cheers,
Danny