Currently, when use Relative Date Filter or Slicer, when you want relative to calendar week, month or year, is possible use three operators "is in the last", "is in this" and "is in the next", which mean "<", "=" and ">". There completely missing ">=" and "<=" ("is in the last or in this" and "is in this or in the next") comparation.
Additionally, is not possible combine (with OR) these comparation (is not possible set "is in the last" n " "calendar months" OR "is in this" "calendar month", as is possible in advanced filter.
And last chance, in slicer is configuration option "anchor date" which can move the relative date and solve this issue, but this option is possible set only to fixed value. Is not possible set dynamically, e.g. = dateadd(today(),1,MONTH). Truly, if static only, I don't know what good is for (for relative date slicer).
Result? Currently is not possible filter date relative as common user want. Is not possible filter dashboard, and see e.g. last 12 whole months in row include current month.
But I hope, it is only bug, and will be soon fixed by adding well know comparators ">=" and "<="