What you want is not a dynamic date slicer but a static date slicer - you want the dates to always be governed by the same calculation. I believe there are a couple of open ideas for this feature - go vote for them.
Do you actually need the date slicer? You can calculate your date range in a calculated column, for example you can add a flag into your dates/calendar table that identifies the dates you want included. And then you can use that flag to drive the slicer, or the actual data display filter.
