   Returns true if the given year is a leap year.
 
   A leap year is a year of a longer length than normal. The exact meaning
-  is up to the calendar. A calendar must return `false` if does not support
+  is up to the calendar. A calendar must return `false` if it does not support
   the concept of leap years.
   """
   @callback leap_year?(year) :: boolean

  @type std_offset  :: integer

  @doc """
  Builds a new date from propletic year, month and day of month.
  """
  @callback date(year, month, day) :: {:ok, Date.t} | {:error, atom}


  @doc """
  Parses the extended "Date and time of day" format described by ISO8601:2004.

  Raises if the format is invalid.

  ## Examples

