From e45c2b07b531e7a0701f0a92b7d2ef6b3a69e578 Mon Sep 17 00:00:00 2001
From: Joshua Chumbley <jchumbley@protonmail.com>
Date: Sat, 19 Mar 2022 14:03:53 -0500
Subject: [PATCH 1/2] Interpret unknown timezones as UTC

---
 lib/icalendar/util/deserialize.ex | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/lib/icalendar/util/deserialize.ex b/lib/icalendar/util/deserialize.ex
index ec65ea4..4f17212 100644
--- a/lib/icalendar/util/deserialize.ex
+++ b/lib/icalendar/util/deserialize.ex
@@ -236,12 +236,7 @@ defmodule ICalendar.Util.Deserialize do
   def to_date(date_string, %{"TZID" => timezone}) do
     # Microsoft Outlook calendar .ICS files report times in Greenwich Standard Time (UTC +0)
     # so just convert this to UTC
-    timezone =
-      if Regex.match?(~r/\//, timezone) do
-        timezone
-      else
-        Timex.Timezone.Utils.to_olson(timezone)
-      end
+    timezone = parse_timezone(timezone)
 
     date_string =
       case String.last(date_string) do
@@ -264,6 +259,21 @@ defmodule ICalendar.Util.Deserialize do
     to_date(date_string, %{"TZID" => "Etc/UTC"})
   end
 
+  defp parse_timezone(timezone) do
+    tz =
+      if Regex.match?(~r/\//, timezone) do
+        timezone
+      else
+        Timex.Timezone.Utils.to_olson(timezone)
+      end
+
+    if is_nil(tz) do
+      "Etc/UTC"
+    else
+      tz
+    end
+  end
+
   defp to_geo(geo) do
     geo
     |> desanitized()

From 545250f61cbc7452e8304e5baecef417310b19a9 Mon Sep 17 00:00:00 2001
From: Joshua Chumbley <jchumbley@protonmail.com>
Date: Sat, 19 Mar 2022 14:08:10 -0500
Subject: [PATCH 2/2] Add test for unrecognized timezone parsing

---
 test/icalendar/deserialize_test.exs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/test/icalendar/deserialize_test.exs b/test/icalendar/deserialize_test.exs
index 5fcd87d..4bf8ded 100644
--- a/test/icalendar/deserialize_test.exs
+++ b/test/icalendar/deserialize_test.exs
@@ -85,6 +85,19 @@ defmodule ICalendar.DeserializeTest do
       assert event.dtend.time_zone == "America/Chicago"
     end
 
+    test "with unrecognized Timezone" do
+      ics = """
+      BEGIN:VEVENT
+      DTEND;TZID=GMT-0500:22221224T084500
+      DTSTART;TZID=GMT-0500:22221224T083000
+      END:VEVENT
+      """
+
+      [event] = ICalendar.from_ics(ics)
+      assert event.dtstart.time_zone == "Etc/UTC"
+      assert event.dtend.time_zone == "Etc/UTC"
+    end
+
     test "with CR+LF line endings" do
       ics = """
       BEGIN:VEVENT