I believe the MDX query is actually using a formatted version of the date, rather than the datetime value passed via the calendar date-picker. I resolved this in my reportby editing the data set - in the parameters tab, I formatted the date by setting the value of the Parameter to:
="[DATE DIMENSION].[Date].&[" & Format(CDate(Parameters!FromDATEDIMENSIONDate.Value),"yyyy-MM-dd") & "T00:00:00]"
My date dimension is called DATE DIMENSION, and the parameter name is "FromDATEDIMENSIONDate"
Hope this helps.
-Marianne