In D365 when you view a Financial Report you can click "Report Options" and select your Base Period/Base Year.
What it should also show is the date that the data will actually be generated for.
If you are running a non-calendar year or had to create an extra year (2016A) in D365 MR would treat that as 2017. Just because you are selecting 10/2016 in the Web Viewer doesn't mean that it is really 10/2016 in D365. In D365 10/2016 could be 6/2016.
When you are in Report Designer and select the Base Period/Base Year it then shows you a "Report Date" section that corresponds to the date in D365.