diff --git a/README.md b/README.md
index 038c97d..bb8dbe5 100755
--- a/README.md
+++ b/README.md
@@ -41,6 +41,9 @@ Or install it yourself as:
 @cal.yahoo_url
 #=> "https://calendar.yahoo.com/?v=60&view=d&type=20&title=Christmas%20party%21&st=20201212T133000Z&dur=0100"
 
+@cal.hey_url
+#=> "https://app.hey.com/calendar/ical_events/new?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT%0ASUMMARY%3AHolly%27s%209th%20birthday%21%0ADTSTAMP%3A20240913T151029Z%0ADTSTART%3A20240906T123000Z%0ADTEND%3A20240906T133000Z%0AUID%3A-20240906T123000Z-Holly%27s%209th%20birthday%21%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
 @cal.office365_url
 #=> "https://outlook.office.com/calendar/0/deeplink/compose?path=/calendar/action/compose&rru=addevent&subject=Christmas%20party%21&startdt=2020-12-12T13:30:00Z&enddt=2020-12-12T14:30:00Z"
 
diff --git a/lib/add_to_calendar.rb b/lib/add_to_calendar.rb
index eba1240..6536c04 100755
--- a/lib/add_to_calendar.rb
+++ b/lib/add_to_calendar.rb
@@ -92,6 +92,59 @@ def yahoo_url
       return calendar_url
     end
 
+    def hey_url
+      calendar_url = "https://app.hey.com/calendar/ical_events/new?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT"
+      
+      params = {}
+      params[:SUMMARY] = url_encode(title)
+      params[:DTSTAMP] = Time.now.strftime("%Y%m%dT%H%M%SZ")
+  
+      if all_day
+        one_day = 1 * 24 * 60 * 60
+        params["DTSTART%3BVALUE=DATE"] = format_date(start_datetime)
+        if end_datetime
+          params["DTEND%3BVALUE=DATE"] = format_date(end_datetime + one_day)
+        else
+          params["DTEND%3BVALUE=DATE"] = format_date(start_datetime + one_day)
+        end
+      else
+        params[:DTSTART] = utc_datetime(start_datetime)
+        if end_datetime
+          params[:DTEND] = utc_datetime(end_datetime)
+        else
+          params[:DTEND] = utc_datetime(start_datetime + 60*60) # 1 hour later
+        end
+      end
+      
+      params[:URL] = url_encode(url) if url
+      params[:UID] = "-#{utc_datetime(start_datetime)}-#{url_encode(title)}"
+      params[:DESCRIPTION] = url_encode_hey(description) if description
+      if add_url_to_description && url
+        if params[:DESCRIPTION]
+          params[:DESCRIPTION] << "\n\n#{url_encode(url)}"
+        else
+          params[:DESCRIPTION] = url_encode(url)
+        end
+      end
+      if organizer
+        params[:ORGANIZER] = url_encode("CN=\"#{organizer[:name]}\"%3Amailto%3A#{organizer[:email]}")
+      end
+      params[:LOCATION] = url_encode(location) if location
+      
+      new_line = "%0A"
+      params.each do |key, value|
+        if key == :ORGANIZER
+          calendar_url << "#{new_line}#{key}%3B#{value}"
+        else
+          calendar_url << "#{new_line}#{key}%3A#{value}"
+        end
+      end
+  
+      calendar_url << "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+    
+      return calendar_url
+    end
+
     def office365_url
       # Eg. https://outlook.live.com/calendar/0/deeplink/compose?path=/calendar/action/compose&rru=addevent&subject=Holly%27s%208th%20Birthday%21&startdt=2020-05-12T12:30:00Z&enddt=2020-05-12T16:00:00Z&body=Come%20join%20us%20for%20lots%20of%20fun%20%26%20cake%21%0A%0Ahttps%3A%2F%2Fwww.example.com%2Fevent-details&location=Flat%204%2C%20The%20Edge%2C%2038%20Smith-Dorrien%20St%2C%20London%2C%20N1%207GU
       microsoft("office365")
@@ -326,6 +379,17 @@ def url_encode_ical(s)
         }.join("\\n")
       end
 
+      def url_encode_hey(s)
+        string = s.dup # don't modify original input
+        string.split("\n").map { |e|
+          if e.empty?
+            e
+          else
+            url_encode(e)
+          end
+        }.join("\n")
+      end
+
       def yahoo_param(key)
         if key == :in_loc
           key.to_s
diff --git a/test/support/assertions/hey_url_assertions.rb b/test/support/assertions/hey_url_assertions.rb
new file mode 100644
index 0000000..ee7d02b
--- /dev/null
+++ b/test/support/assertions/hey_url_assertions.rb
@@ -0,0 +1,68 @@
+require 'uri'
+require 'cgi'
+
+module Minitest::Assertions
+  def assert_hey_urls_equal(expected, actual, msg = nil)
+    differences = compare_hey_urls(expected, actual)
+    
+    if differences.empty?
+      assert true # URLs are identical
+    else
+      failure_message = build_failure_message(differences)
+      msg = message(msg) { failure_message }
+      assert false, msg
+    end
+  end
+
+  private
+
+  def decode_ical_source(url)
+    uri = URI(url)
+    CGI.unescape(URI.decode_www_form(uri.query).to_h['ical_source']).split("\n")
+  end
+
+  def compare_hey_urls(url1, url2)
+    ical1 = decode_ical_source(url1)
+    ical2 = decode_ical_source(url2)
+  
+    differences = []
+  
+    # Compare each line of the decoded ical_source
+    ical1.zip(ical2).each_with_index do |(line1, line2), index|
+      if line1 != line2
+        differences << {
+          line_number: index + 1,
+          expected_line: line2,
+          actual_line: line1
+        }
+      end
+    end
+  
+    # Handle case where one ical_source is longer than the other
+    if ical1.length != ical2.length
+      start = [ical1.length, ical2.length].min
+      longer = ical1.length > ical2.length ? ical1 : ical2
+      longer_label = ical1.length > ical2.length ? 'actual' : 'expected'
+  
+      longer[start..-1].each_with_index do |line, i|
+        differences << {
+          line_number: start + i + 1,
+          "#{longer_label}_line": line,
+          "#{longer_label == 'actual' ? 'expected_line' : 'actual_line'}": nil
+        }
+      end
+    end
+  
+    differences
+  end  
+
+  def build_failure_message(differences)
+    message = "Hey URLs do not match. Differences found:\n"
+    differences.each do |diff|
+      message << "Line #{diff[:line_number]}:\n"
+      message << "  Expected: #{diff[:expected_line]}\n"
+      message << "  Actual:   #{diff[:actual_line]}\n\n"
+    end
+    message
+  end
+end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 4e9def7..5435002 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -3,3 +3,5 @@
 require "minitest/autorun"
 require "timecop"
 require "pry"
+
+Dir[File.expand_path('support/**/*.rb', __dir__)].each { |file| require file }
diff --git a/test/urls/hey_url_test.rb b/test/urls/hey_url_test.rb
new file mode 100644
index 0000000..67dfb97
--- /dev/null
+++ b/test/urls/hey_url_test.rb
@@ -0,0 +1,488 @@
+require "test_helper"
+
+class HeyUrlTest < Minitest::Test
+  def setup
+    # We need to freeze time on each test because DTSTAMP is generated via Time.now
+    Timecop.freeze(Time.now.utc)
+  end
+
+  def teardown
+    Timecop.return
+  end
+  
+  def test_without_end_datetime_ends_one_hour_after_start
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      title: event.title, 
+      timezone: event.timezone
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+               "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+               "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+               "%0ASUMMARY%3A#{event.title_encoded}" + 
+               "%0ADTSTAMP%3A#{event.dtstamp}" + 
+               "%0ADTSTART%3A#{event.dtstart}" + 
+               "%0ADTEND%3A#{event.dtstart_plus_one_hour}" + 
+               event.uid + 
+               "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_with_end_datetime
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime: Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+  
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone
+    )
+  
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+  
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_with_end_datetime_after_midnight
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,23,30,00,0),
+      end_datetime:   Time.new(2024,9,7,1,30,00,0),
+      title: "Holly's 19th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+    
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_with_location
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      location: "Address, Postcode"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone, 
+      location: event.location
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                event.uid + 
+                "%0ALOCATION%3A#{event.location_encoded}" +
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_with_url_without_description
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      url: "https://www.example.com/event-details"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone,
+      url: event.url
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                "%0AURL%3A#{event.url_encoded}" +
+                event.uid + 
+                "%0ADESCRIPTION%3A#{event.url_encoded}" +
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_with_url_and_description
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      url: "https://www.example.com/event-details",
+      description: "Come join us for lots of fun & cake!"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone,
+      url: event.url,
+      description: event.description
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                "%0AURL%3A#{event.url_encoded}" +
+                event.uid + 
+                "%0ADESCRIPTION%3A#{event.description_encoded}\n\n#{event.url_encoded}" +
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_description_with_newlines
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      description: "Test\n\nNewline"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone,
+      url: event.url,
+      description: event.description
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                event.uid + 
+                "%0ADESCRIPTION%3A#{"Test\n\nNewline"}" +
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_add_url_to_description_false_without_url
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone,
+      add_url_to_description: false
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_add_url_to_description_false_with_url
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      url: "https://www.example.com/event-details"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title, 
+      timezone: event.timezone,
+      url: event.url,
+      add_url_to_description: false
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                "%0AURL%3A#{event.url_encoded}" +
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+  
+  def test_with_all_attributes
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC",
+      location: "Address, Postcode",
+      url: "https://www.example.com/event-details",
+      description: "Come join us for lots of fun & cake!"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime,
+      end_datetime: event.end_datetime,
+      title: event.title, 
+      timezone: event.timezone,
+      url: event.url,
+      location: event.location,
+      description: event.description,
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3A#{event.dtstart}" + 
+                "%0ADTEND%3A#{event.dtend}" + 
+                "%0AURL%3A#{event.url_encoded}" +
+                event.uid + 
+                "%0ADESCRIPTION%3A#{event.description_encoded}\n\n#{event.url_encoded}" +
+                "%0ALOCATION%3A#{event.location_encoded}" +
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_all_day_spans_single_day
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title,
+      timezone: event.timezone,
+      all_day: true
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3BVALUE%3DDATE%3A#{"20240906"}" + 
+                "%0ADTEND%3BVALUE%3DDATE%3A#{"20240907"}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_all_day_spans_multiple_days
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,13,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title,
+      timezone: event.timezone,
+      all_day: true
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3BVALUE%3DDATE%3A#{"20240906"}" + 
+                "%0ADTEND%3BVALUE%3DDATE%3A#{"20240914"}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_all_day_without_end_date_is_single_day
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      title: event.title,
+      timezone: event.timezone,
+      all_day: true
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3BVALUE%3DDATE%3A#{"20240906"}" + 
+                "%0ADTEND%3BVALUE%3DDATE%3A#{"20240907"}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  def test_all_day_end_date_is_plus_one_from_end_date
+    event = TestEvent.new(
+      start_datetime: Time.new(2024,9,6,12,30,00,0),
+      end_datetime:   Time.new(2024,9,6,15,30,00,0),
+      title: "Holly's 9th birthday!",
+      timezone: "UTC"
+    )
+
+    cal = AddToCalendar::URLs.new(
+      start_datetime: event.start_datetime, 
+      end_datetime: event.end_datetime, 
+      title: event.title,
+      timezone: event.timezone,
+      all_day: true
+    )
+
+    expected = "https://app.hey.com/calendar/ical_events/new" + 
+                "?ical_source=BEGIN%3AVCALENDAR%0AVERSION%3A2.0" +
+                "%0APRODID%3A-//AddToCalendar//RubyGem//EN%0ABEGIN%3AVEVENT" + 
+                "%0ASUMMARY%3A#{event.title_encoded}" + 
+                "%0ADTSTAMP%3A#{event.dtstamp}" + 
+                "%0ADTSTART%3BVALUE%3DDATE%3A#{"20240906"}" + 
+                "%0ADTEND%3BVALUE%3DDATE%3A#{"20240907"}" + 
+                event.uid + 
+                "%0AEND%3AVEVENT%0AEND%3AVCALENDAR"
+
+    assert_hey_urls_equal cal.hey_url, expected
+  end
+
+  private
+
+  class TestEvent
+    attr_reader :start_datetime, :end_datetime, :title, :timezone, :location, :url, :description
+
+    def initialize(start_datetime:, end_datetime: nil, title:, timezone:, location: nil, url: nil, description: nil)
+      @start_datetime = start_datetime
+      @end_datetime = end_datetime
+      @title = title
+      @timezone = timezone
+      @location = location
+      @url = url
+      @description = description
+    end
+
+    def dtstart
+      @start_datetime.utc.strftime('%Y%m%dT%H%M%SZ')
+    end
+
+    def dtstart_plus_one_hour
+      one_hour = 60*60
+      (@start_datetime + one_hour).utc.strftime('%Y%m%dT%H%M%SZ')
+    end
+
+    def dtend
+      @end_datetime.utc.strftime('%Y%m%dT%H%M%SZ')
+    end
+
+    def uid
+      "%0AUID%3A-#{url_encode(dtstart)}-#{url_encode(@title)}"
+    end
+
+    def dtstamp
+      Time.now.utc.strftime('%Y%m%dT%H%M%SZ')
+    end
+
+    def title_encoded
+      url_encode(@title)
+    end
+
+    def location_encoded
+      url_encode(@location)
+    end
+
+    def url_encoded
+      url_encode(@url)
+    end
+
+    def description_encoded
+      url_encode(@description)
+    end
+  end  
+end