As mentioned in these threads: - http://community.powerbi.com/t5/Integrations-with-Files-and/Date-issues-with-Sharepoint-file-sources/m-p/372661#M16100 - https://community.powerbi.com/t5/Desktop/Sh...
I have had the issue regarding dates where the Power BI sharepoint connector is doing some type transformations that cannot be edited.
The issue for me arrises when the sharepoint site and power bi are in different regions that have different date formats. Changing either Power BI or the sharepoint site fixed the issue for me, however this is not possible in all cases. Ideally microsoft can change the sharepoint connector so that is just imports the sharepoint site data as all or text data type so then the end user can set the data types the same way most other connectors operate.