Add timezone handling

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/timezone_detector.ex

@@ -0,0 +1,40 @@
+defmodule Mobilizon.Service.TimezoneDetector do
+  @moduledoc """
+  Detect the timezone from a point
+  """
+
+  @type detectable :: Geo.Point.t() | Geo.PointZ.t() | {float() | float()}
+
+  @doc """
+  Detect the most appropriate timezone from a value, a geographic set of coordinates and a fallback
+  """
+  @spec detect(String.t() | nil, detectable(), String.t()) :: String.t()
+  def detect(nil, geo, fallback) do
+    case TzWorld.timezone_at(geo) do
+      {:ok, timezone} ->
+        timezone
+
+      {:error, :time_zone_not_found} ->
+        fallback
+    end
+  end
+
+  def detect(timezone, geo, fallback) do
+    if Tzdata.zone_exists?(timezone) do
+      timezone
+    else
+      detect(nil, geo, fallback)
+    end
+  end
+
+  @spec detect(String.t() | nil, String.t()) :: String.t()
+  def detect(nil, fallback), do: fallback
+
+  def detect(timezone, fallback) do
+    if Tzdata.zone_exists?(timezone) do
+      timezone
+    else
+      fallback
+    end
+  end
+end