QUARTER() isn't a calc either.
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I'd bet that it has something to do with the queries Tableau generates. I'd bet that most, if not all, data sources have YEAR, MONTH, and DAY functions but are missing functions for WEEK and QUARTER. This is why we then have to implement them in Tableau with datepart('week',[Date]) calcs which then produce a specific query that uses the existing information with some date math in the queries to create weeks and quarters.
All speculation on my part but I'd bet it's close . Maybe....