Timeline Slicer custom fiscal period

I am using Excel's 2013 timeline slicer, and need to filter by month(s). The problem is that the monthly reporting period begins on the 15th of the month. The user insists on using the timeline slicer, and does not wish to use the drop down filter on the PivotTable, where he would have to choose the 'Between' option, then set the first & last day. The user will either filter by a single month (e.g. Nov 15 - Dec 14), or by multiple consecutive months (e.g. Nov 15 - Feb 14).

Here is what I would expect from each of the Timeline Time Levels:
YEARS - can behave as it normally does, but I am not against this filtering to show the fiscal year
QUARTERS - (this filter option will not be used) can behave as it normally does, but I am not against this filtering to show the fiscal year
MONTHS - I would suggest that in my workbook I would have a named range (cell) that accepts my starting day of the month (e.g. 15) Hopefully this could be used as a variable in the code that may be required.
DAYS - can behave as it normally does

I should have also mentioned that I a filtering a PivotTable. I tried to create a calculated field, but since I am using a Timeline slicer (new to 2013), it will accept only dates. The calculated column was not perceived as a date, therefore could not be used for the Timeline slicer. Maybe if I tried to add the column to the data before it is summarized in the PivotTable ...