   @doc """
   Possible time zone periods for a certain time zone and wall clock date and time.
 
-  When the provided `datetime` is ambiguous a tuple with `:ambiguous` and two possible
-  periods. The periods in the list are sorted with the first element being the one that begins first.
+  When the provided naive datetime is ambiguous, return a tuple with `:ambiguous`
+  and the two possible periods. The periods in the tuple must be sorted with the
+  first element being the one that begins first.
 
-  When the provided `datetime` is in a gap - for instance during the "spring forward" when going
-  from winter time to summer time, a tuple with `:gap` and two periods with limits are returned
+  When the provided naive datetime is in a gap, such as during the "spring forward" when going
+  from winter time to summer time, return a tuple with `:gap` and two periods with limits
   in a nested tuple. The first nested two-tuple is the period before the gap and a naive datetime
   with a limit for when the period ends (wall time). The second nested two-tuple is the period
   just after the gap and a datetime (wall time) for when the period begins just after the gap.
 
-  If there is only a single possible period for the provided `datetime`, then a tuple with `:ok`
-  and the `time_zone_period` is returned.
+  If there is only a single possible period for the provided `datetime`, then return a tuple
+  with `:ok` and the `time_zone_period`.
   """
   @doc since: "1.8.0"
   @callback time_zone_periods_from_wall_datetime(Calendar.naive_datetime(), Calendar.time_zone()) ::


defmodule Calendar.UTCOnlyTimeZoneDatabase do
  @moduledoc """
  Built-in time zone database that works only in Etc/UTC.

  For all other time zones, it returns `{:error, :utc_only_time_zone_database}`.
  """

  """

  @typedoc """
  A period where a certain combination of UTC offset, standard offset, and zone
  abbreviation is in effect.

  For example, one period could be the summer of 2018 in the `Europe/London` timezone,
  where summer time/daylight saving time is in effect and lasts from spring to autumn.
  In autumn, the `std_offset` changes along with the `zone_abbr` so a different
  period is needed during winter.
  """
  @type time_zone_period :: %{
          optional(any) => any,
