Time zone handling from postgres to tableau

Hi,

I have already looked at this article and it does not solve my purpose, hence i am reaching out on this forum: Timestamp with Time Zone Converted to UTC in PostgreSQL | Tableau Software

I have a timestamptz column (let's say date_A) in postgres (which i believe is storing time in UTC) and then i have another date column (date_B) which stores date (no timestamp and no timestamptz, just pure date). How do i do a difference in days calculation on this?

I cannot convert the local date (date_B) into UTC because i do not know the source time zone of date_B. I cannot convert date_A to a specific timezone because date_B could be in any timezone.