Slicer-based date range used as parameters for calculated column

My calendar table starts on 9/1/2017 and ends on 4/30/2019.  I am simply trying create a calculated column that indicates if the date is either in the BASE period or in the TEST period.  The BASE period runs from 9/1/2017 to 6/30/2018 and the TEST period runs from 7/1/2018 to 4/30/2019.  What is wrong with my calculated column because it returns TEST PERIOD for all the dates including those which are part of the base period?

=if(('Calendar'[Date] >= 9/1/2017 && 'Calendar'[Date] <= 6/30/2018), "Base Period", "Test Period")

Thank you.