Appears to be daylight savings issue according from details established on other peoples posts, listed at end. I am on Australian Eastern Server and my Time Zone is currently AEDT (Melbourne based). When Power BI receives 1st October 2017 it is changing it to 30th September 2017, this obviously is skewing results in Power BI.
I have tried converting it in many forms as a workaround such as directly from text etc yyyymmdd but on preview in desktop the queries window shows correct date, however once in either Power BI desktop or app.powerbi.com the date is appearing as 30 Sept 2017.
Other users encountering this on desktop (is now occuring on app.powerbi.com also), I can confirm that this bug has only recently made its way to app.powerbi.com