Add time zone to ical

Gemfile

@@ -64,7 +64,7 @@ gem "rqrcode"
 
 gem "draper"
 
-gem "icalendar"
+gem "icalendar", require: ['icalendar', 'icalendar/tzinfo']
 
 group :development do
   gem "spring"

app/views/api/events/index.ics.erb

@@ -1,16 +1,17 @@
 <%
 cal = Icalendar::Calendar.new
+cal.add_timezone(Time.zone.tzinfo.ical_timezone(Time.now))
 
 @events.each do |event|
     next unless event&.slot&.starts_at
 
     cal.event do |ical_event|
-    ical_event.dtstart       = event.slot.starts_at
-    ical_event.dtend         = event.slot.ends_at
+        ical_event.dtstart       = Icalendar::Values::DateTime.new(event.slot.starts_at, 'tzid' => event.slot.starts_at.time_zone.tzinfo.identifier)
+        ical_event.dtend         = Icalendar::Values::DateTime.new(event.slot.ends_at, 'tzid' => event.slot.ends_at.time_zone.tzinfo.identifier)
         ical_event.summary       = event.title
         ical_event.description   = event.description
-    ical_event.created       = event.created_at
-    ical_event.last_modified = event.updated_at
+        ical_event.created       = Icalendar::Values::DateTime.new(event.created_at, 'tzid' => event.created_at.time_zone.tzinfo.identifier)
+        ical_event.last_modified = Icalendar::Values::DateTime.new(event.updated_at, 'tzid' => event.updated_at.time_zone.tzinfo.identifier)
         ical_event.location      = event.slot.hall.name
         ical_event.url           = "https://www.openfest.org/2019/bg/full-schedule-bg/#lecture-#{event.id}"
         ical_event.alarm do |alarm|