From c4e8c70e7d2d7bea703f34f68f05acc2f1ff8572 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Sun, 1 Feb 2026 11:02:57 -0600
Subject: [PATCH] Add CaptureTimezone plug to extract cf-timezone header

---
 lib/towerops_web/plugs/capture_timezone.ex    | 25 ++++++++++
 .../plugs/capture_timezone_test.exs           | 48 +++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 lib/towerops_web/plugs/capture_timezone.ex
 create mode 100644 test/towerops_web/plugs/capture_timezone_test.exs

diff --git a/lib/towerops_web/plugs/capture_timezone.ex b/lib/towerops_web/plugs/capture_timezone.ex
new file mode 100644
index 00000000..adae2ead
--- /dev/null
+++ b/lib/towerops_web/plugs/capture_timezone.ex
@@ -0,0 +1,25 @@
+defmodule ToweropsWeb.Plugs.CaptureTimezone do
+  @moduledoc """
+  Captures the cf-timezone header from Cloudflare and stores it in session.
+
+  Cloudflare provides timezone detection via the cf-timezone header using
+  IANA timezone database format (e.g., "America/Chicago", "Europe/London").
+
+  Falls back to "UTC" if the header is not present (development, non-Cloudflare).
+  """
+  import Plug.Conn
+
+  @doc false
+  def init(opts), do: opts
+
+  @doc false
+  def call(conn, _opts) do
+    timezone =
+      case get_req_header(conn, "cf-timezone") do
+        [tz] when is_binary(tz) and byte_size(tz) > 0 -> tz
+        _ -> "UTC"
+      end
+
+    put_session(conn, :detected_timezone, timezone)
+  end
+end
diff --git a/test/towerops_web/plugs/capture_timezone_test.exs b/test/towerops_web/plugs/capture_timezone_test.exs
new file mode 100644
index 00000000..764d08ba
--- /dev/null
+++ b/test/towerops_web/plugs/capture_timezone_test.exs
@@ -0,0 +1,48 @@
+defmodule ToweropsWeb.Plugs.CaptureTimezoneTest do
+  use ToweropsWeb.ConnCase, async: true
+
+  alias ToweropsWeb.Plugs.CaptureTimezone
+
+  describe "call/2" do
+    test "captures cf-timezone header and stores in session", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> put_req_header("cf-timezone", "America/Chicago")
+        |> CaptureTimezone.call([])
+
+      assert get_session(conn, :detected_timezone) == "America/Chicago"
+    end
+
+    test "defaults to UTC when cf-timezone header is missing", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> CaptureTimezone.call([])
+
+      assert get_session(conn, :detected_timezone) == "UTC"
+    end
+
+    test "defaults to UTC when cf-timezone header is empty", %{conn: conn} do
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> put_req_header("cf-timezone", "")
+        |> CaptureTimezone.call([])
+
+      assert get_session(conn, :detected_timezone) == "UTC"
+    end
+
+    test "handles overridden timezone headers by taking last", %{conn: conn} do
+      # put_req_header replaces the header value, so the last one wins
+      conn =
+        conn
+        |> init_test_session(%{})
+        |> put_req_header("cf-timezone", "America/Chicago")
+        |> put_req_header("cf-timezone", "Europe/London")
+        |> CaptureTimezone.call([])
+
+      assert get_session(conn, :detected_timezone) == "Europe/London"
+    end
+  end
+end