This "civic vs proleptic" split is a good use case for composite calendars: glue a few calendars together at their historical cut-over dates and the result behaves like one ordinary calendar.

I maintain an Elixir library (Calendrical) that does this. England, for example, is just three segments spliced onto a Julian base:

  defmodule England do
    use Calendrical.Composite,
      calendars: [
        ~D[1155-03-25 Calendrical.Julian.March25],  # year starts on Lady Day
        ~D[1751-03-25 Calendrical.Julian.Jan1],     # New Year moved to Jan 1
        ~D[1752-09-14 Calendrical.Gregorian]        # the 11-day jump
      ],
      base_calendar: Calendrical.Julian
  end

The "missing" days simply aren't valid dates, and stepping across the boundary jumps:

  England.days_in_month(1752, 9)              #=> 19
  England.valid_date?(1752, 9, 3)             #=> false
  Date.shift(~D[1752-09-02 England], day: 1)  #=> ~D[1752-09-14 England]