Hopefully, I am missing some basic functionality, but after reading multiple threads on the topic, I am losing hope there is already a quick fix.
Tableau Prep currently doesn't seem to have the "set fiscal year start" on a dataset.
This should be standard functionality as it is for a workbook in Tableau Desktop. Having to reset it with every workbook, every time I reopen to connect to a Tableau Online datasource is crazy, and writing in formulae to establish this is also impractical.
Ideally, this could additionally be set in the Tableau Online/Tableau Server interface.