Find the right time zone for standard times or daylight saving times

Using `.now` causes a search for "MST" in the winter, even if the time
of the given schedule is in summer and would only match "MDT". By
parsing the time value and using the components as values in the
candidate time zone, this issue is fixed.

Author: epologee

lib/ice_cube/parsers/ical_parser.rb

@@ -5,7 +5,7 @@ def self.schedule_from_ical(ical_string, options = {})
       ical_string.each_line do |line|
         (property, value) = line.split(":")
         (property, tzid) = property.split(";")
-        zone = find_zone(tzid) if tzid.present?
+        zone = find_zone(tzid, value) if tzid.present?
         case property
         when "DTSTART"
           value = { time: value, zone: zone } if zone.present?
@@ -95,13 +95,15 @@ def self.rule_from_ical(ical)
 
     private
 
-    def self.find_zone(tzid)
+    def self.find_zone(tzid, time_string)
       (_, zone) = tzid&.split("=")
       begin
         Time.find_zone!(zone) if zone.present?
       rescue ArgumentError
         (rails_zone, _tzinfo_id) = ActiveSupport::TimeZone::MAPPING.find do |(k, _)|
-          Time.find_zone!(k).now.strftime("%Z") == zone
+          time = Time.parse(time_string)
+
+          Time.find_zone!(k).local(time.year, time.month, time.day, time.hour, time.min).strftime("%Z") == zone
         end
 
         Time.find_zone(rails_zone)

spec/examples/from_ical_spec.rb

@@ -378,7 +378,7 @@ def sorted_ical(ical)
         end
       end
 
-      it "round trips from and to ical with time zones" do
+      it "round trips from and to ical with time zones in the summer (MDT)" do
         original = <<-ICAL.gsub(/^\s*/, "").strip
           DTSTART;TZID=MDT:20130731T143000
           RRULE:FREQ=WEEKLY;UNTIL=20140730T203000Z;BYDAY=MO,WE,FR
@@ -393,6 +393,22 @@ def sorted_ical(ical)
         schedule_from_ical = IceCube::Schedule.from_ical original
         expect(schedule_from_ical.to_ical).to eq original
       end
+
+      it "round trips from and to ical with time zones in the winter (MST)" do
+        original = <<-ICAL.gsub(/^\s*/, "").strip
+          DTSTART;TZID=MST:20130131T143000
+          RRULE:FREQ=WEEKLY;UNTIL=20140130T203000Z;BYDAY=MO,WE,FR
+          RDATE;TZID=MST:20150212T143000
+          RDATE;TZID=MST:20150207T143000
+          EXDATE;TZID=MST:20130223T143000
+          EXDATE;TZID=MST:20130212T143000
+          EXDATE;TZID=MST:20130207T143000
+          DTEND;TZID=MST:20130131T153000
+        ICAL
+
+        schedule_from_ical = IceCube::Schedule.from_ical original
+        expect(schedule_from_ical.to_ical).to eq original
+      end
     end
 
     describe "exceptions" do