Specs

plus(
  year :: Calendar.year(),
  month :: Calendar.month() | week(),
  day :: Calendar.day(),
  months_or_quarters :: :months | :quarters,
  increment :: integer(),
  options :: Keyword.t()
) :: {Calendar.year(), Calendar.month(), Calendar.day()}

Increments a Date.t or Date.Range.t by a specified positive or negative integer number of periods (year, quarter, month, week or day).

Calendars need only implement this callback for :months and :quarters since all other date periods can be derived.