There seems to be disagreement about the year. Two systems I checked assume 1752, not 1582.

(OpenBSD)

  $ man cal
  [...]
  The Gregorian Reformation is assumed to have occurred in
  1752 after the 2nd of September.  By this time, most
  countries had recognized the Reformation (although a few
  did not recognize it until the early 1900s). Eleven days
  following that date were eliminated by the Reformation, so
  the calendar for that month is a bit unusual.

(Ubuntu Linux)

  [...]
  -s country_code
      Assume the switch from Julian to Gregorian Calendar at
      the date associated with the country_code.  If not
      specified, ncal tries to guess the switch date from the
      local environment or falls back to September 2, 1752.
      This was when Great Britain and her colonies switched
      to the Gregorian Calendar.