feat: add timezone handling

lib/mv_web/helpers/date_formatter.ex

@@ -2,6 +2,7 @@ defmodule MvWeb.Helpers.DateFormatter do
   @moduledoc """
   Centralized date formatting helper for the application.
   Formats dates in European format (dd.mm.yyyy).
+  DateTime can be shown in UTC or in a given IANA timezone (e.g. from browser).
   """
 
   use Gettext, backend: MvWeb.Gettext
@@ -28,19 +29,40 @@ defmodule MvWeb.Helpers.DateFormatter do
   @doc """
   Formats a DateTime struct to European format (dd.mm.yyyy HH:MM).
 
+  When `timezone` is a valid IANA timezone string (e.g. from the browser),
+  the datetime is converted to that zone before formatting. When `timezone` is
+  nil or invalid, the datetime is formatted in UTC.
+
   ## Examples
 
       iex> MvWeb.Helpers.DateFormatter.format_datetime(~U[2024-03-15 10:30:00Z])
       "15.03.2024 10:30"
 
+      iex> MvWeb.Helpers.DateFormatter.format_datetime(~U[2024-03-15 10:30:00Z], "Europe/Berlin")
+      "15.03.2024 11:30"
+
       iex> MvWeb.Helpers.DateFormatter.format_datetime(nil)
       ""
   """
-  def format_datetime(%DateTime{} = dt) do
-    Calendar.strftime(dt, "%d.%m.%Y %H:%M")
+  def format_datetime(%DateTime{} = dt), do: format_datetime(dt, nil)
+  def format_datetime(nil), do: ""
+  def format_datetime(_), do: "Invalid datetime"
+
+  def format_datetime(%DateTime{} = dt, nil), do: format_datetime_utc(dt)
+  def format_datetime(%DateTime{} = dt, ""), do: format_datetime_utc(dt)
+
+  def format_datetime(%DateTime{} = dt, tz) when is_binary(tz) do
+    case DateTime.shift_zone(dt, tz, Tz.TimeZoneDatabase) do
+      {:ok, shifted} -> Calendar.strftime(shifted, "%d.%m.%Y %H:%M")
+      {:error, _} -> format_datetime_utc(dt)
+    end
   end
 
-  def format_datetime(nil), do: ""
+  def format_datetime(nil, _timezone), do: ""
 
-  def format_datetime(_), do: "Invalid datetime"
+  def format_datetime(_, _timezone), do: "Invalid datetime"
+
+  defp format_datetime_utc(%DateTime{} = dt) do
+    Calendar.strftime(dt, "%d.%m.%Y %H:%M")
+  end
 end