The Data Provider transformation "Date String" is undefined or is not effective and has no documentation. Step to reproduce: 1) Open Data Provider and load a source with date contents 2) Select Date and Time Transformation 3) Select "Date String" 4) Click Apply Actual Results (so far observed): nothing happens Expected result: something happens in Preview area. Source code: https://opengrok.libreoffice.org/xref/core/sc/source/ui/dataprovider/datatransformation.cxx?#717 Help page: https://help.libreoffice.org/master/en-US/text/scalc/01/data_provider.html?DbPAR=CALC
From the code, DATE_STRING was supposed to make sure the column is formatted as a Date. For some reason, it does not seem to work. My guess is that ScOutputData::LayoutStrings does not account for the SvNumFormatType::DATE format.
One interesting side effect: Right-click on the column header of the preview table and it selects the corresponding column in the database range. Then you can format the cells with the context menu that opens in the preview table.