diff --git a/test/support/calendar/holocene.ex b/test/support/calendar/holocene.ex
new file mode 100644
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--- /dev/null
+++ b/test/support/calendar/holocene.ex
@@ -0,0 +1,177 @@
+defmodule Calendar.Holocene do
+  # This calendar is used for tests with an none ISO calendar.
+  # It implements the Holocene calendar, which is based on the
+  # Proleptic Gregorian calendar with every year + 10_000.
+
+  @behaviour Calendar
+
+  def date(year, month, day) do
+    %Date{year: year, month: month, day: day, calendar: __MODULE__}
+  end
+
+  def naive_datetime(year, month, day, hour, minute, second, microsecond \\ {0, 0}) do
+    %NaiveDateTime{
+      year: year,
+      month: month,
+      day: day,
+      hour: hour,
+      minute: minute,
+      second: second,
+      microsecond: microsecond,
+      calendar: __MODULE__
+    }
+  end
+
+  @impl true
+  def date_to_string(year, month, day) do
+    "#{year}-#{zero_pad(month, 2)}-#{zero_pad(day, 2)}"
+  end
+
+  @impl true
+  def naive_datetime_to_string(year, month, day, hour, minute, second, microsecond) do
+    "#{year}-#{zero_pad(month, 2)}-#{zero_pad(day, 2)} " <>
+      Calendar.ISO.time_to_string(hour, minute, second, microsecond)
+  end
+
+  @impl true
+  def datetime_to_string(
+        year,
+        month,
+        day,
+        hour,
+        minute,
+        second,
+        microsecond,
+        time_zone,
+        zone_abbr,
+        utc_offset,
+        std_offset
+      ) do
+    "#{year}-#{zero_pad(month, 2)}-#{zero_pad(day, 2)} " <>
+      Calendar.ISO.time_to_string(hour, minute, second, microsecond, :extended) <>
+      Calendar.ISO.offset_to_string(utc_offset, std_offset, time_zone, :extended) <>
+      zone_to_string(utc_offset, std_offset, zone_abbr, time_zone)
+  end
+
+  defp zone_to_string(_, _, _, "Etc/UTC"), do: ""
+  defp zone_to_string(_, _, abbr, zone), do: " " <> abbr <> " " <> zone
+
+  @impl true
+  defdelegate time_to_string(hour, minute, second, microsecond), to: Calendar.ISO
+
+  @impl true
+  def day_rollover_relative_to_midnight_utc(), do: {0, 1}
+
+  @impl true
+  def naive_datetime_from_iso_days(entry) do
+    {year, month, day, hour, minute, second, microsecond} =
+      Calendar.ISO.naive_datetime_from_iso_days(entry)
+
+    {year + 10_000, month, day, hour, minute, second, microsecond}
+  end
+
+  @impl true
+  def naive_datetime_to_iso_days(year, month, day, hour, minute, second, microsecond) do
+    Calendar.ISO.naive_datetime_to_iso_days(
+      year - 10_000,
+      month,
+      day,
+      hour,
+      minute,
+      second,
+      microsecond
+    )
+  end
+
+  defp zero_pad(val, count) when val >= 0 do
+    String.pad_leading("#{val}", count, ["0"])
+  end
+
+  defp zero_pad(val, count) do
+    "-" <> zero_pad(-val, count)
+  end
+
+  @impl true
+  def parse_date(string) do
+    {year, month, day} =
+      string
+      |> String.split("-")
+      |> Enum.map(&String.to_integer/1)
+      |> List.to_tuple()
+
+    if valid_date?(year, month, day) do
+      {:ok, {year, month, day}}
+    else
+      {:error, :invalid_date}
+    end
+  end
+
+  @impl true
+  def valid_date?(year, month, day) do
+    :calendar.valid_date(year, month, day)
+  end
+
+  @impl true
+  def months_in_year(year) do
+    Calendar.ISO.months_in_year(year - 10_000)
+  end
+
+  @impl true
+  def days_in_month(year, month) do
+    Calendar.ISO.days_in_month(year - 10_000, month)
+  end
+
+  @impl true
+  def leap_year?(year) do
+    Calendar.ISO.leap_year?(year - 10_000)
+  end
+
+  @impl true
+  def day_of_week(year, month, day, starting_on) do
+    Calendar.ISO.day_of_week(year - 10_000, month, day, starting_on)
+  end
+
+  @impl true
+  def day_of_year(year, month, day) do
+    Calendar.ISO.day_of_year(year - 10_000, month, day)
+  end
+
+  @impl true
+  def quarter_of_year(year, month, day) do
+    Calendar.ISO.quarter_of_year(year - 10_000, month, day)
+  end
+
+  @impl true
+  def year_of_era(year, month, day) do
+    Calendar.ISO.year_of_era(year - 10_000, month, day)
+  end
+
+  @impl true
+  def day_of_era(year, month, day) do
+    Calendar.ISO.day_of_era(year - 10_000, month, day)
+  end
+
+  @impl true
+  defdelegate parse_time(string), to: Calendar.ISO
+
+  @impl true
+  defdelegate parse_naive_datetime(string), to: Calendar.ISO
+
+  @impl true
+  defdelegate parse_utc_datetime(string), to: Calendar.ISO
+
+  @impl true
+  defdelegate time_from_day_fraction(day_fraction), to: Calendar.ISO
+
+  @impl true
+  defdelegate time_to_day_fraction(hour, minute, second, microsecond), to: Calendar.ISO
+
+  @impl true
+  defdelegate valid_time?(hour, minute, second, microsecond), to: Calendar.ISO
+
+  @impl true
+  defdelegate iso_days_to_beginning_of_day(iso_days), to: Calendar.ISO
+
+  @impl true
+  defdelegate iso_days_to_end_of_day(iso_days), to: Calendar.ISO
+end