Not sure if I understood your problem correctly. Do you have monthly data? And not sure how 10/31/2019 is quarter 4 and converting that to 12/31/2019 which is again the end date of quarter 4.
An attachment would help.
I would first truncate the date to the quarter, so 10/31/2019 would become 10/01/2019.
Then, I would add a quarter so it would become 01/01/2020.
Then, I'd subtract a day so it would become 12/31/2019.
That would work for any date. The calculation would be:
DATEADD('day', -1, DATEADD('quarter', 1, DATETRUNC('quarter', [Date])))
Hope that helps!