diff --git a/app/models/card/activity_spike/detector.rb b/app/models/card/activity_spike/detector.rb
index f879fadf7..aaeb8adc1 100644
--- a/app/models/card/activity_spike/detector.rb
+++ b/app/models/card/activity_spike/detector.rb
@@ -7,8 +7,12 @@ class Card::ActivitySpike::Detector
 
   def detect
     if has_activity_spike?
-      activity_spike = card.activity_spike || card.build_activity_spike
-      activity_spike.touch
+      if card.activity_spike
+        card.activity_spike.touch
+      else
+        card.create_activity_spike!
+      end
+
       true
     else
       false
@@ -22,7 +26,7 @@ class Card::ActivitySpike::Detector
 
     def multiple_people_commented?
       card.comments
-        .where("created_at >= ?", recent_period.ago)
+        .where("created_at >= ?", recent_period.seconds.ago)
         .group(:card_id)
         .having("COUNT(*) >= ?", minimum_comments)
         .having("COUNT(DISTINCT creator_id) >= ?", minimum_participants)
diff --git a/app/models/card/stallable.rb b/app/models/card/stallable.rb
index fefa222f1..e460cd030 100644
--- a/app/models/card/stallable.rb
+++ b/app/models/card/stallable.rb
@@ -13,7 +13,11 @@ module Card::Stallable
   end
 
   def stalled?
-    activity_spike && activity_spike.updated_at < STALLED_AFTER_LAST_SPIKE_PERIOD.ago
+    last_activity_spike_at < STALLED_AFTER_LAST_SPIKE_PERIOD.ago if activity_spike.present?
+  end
+
+  def last_activity_spike_at
+    activity_spike&.updated_at
   end
 
   def detect_activity_spikes
diff --git a/test/models/card/activity_spike/detector_test.rb b/test/models/card/activity_spike/detector_test.rb
new file mode 100644
index 000000000..8c1072e0c
--- /dev/null
+++ b/test/models/card/activity_spike/detector_test.rb
@@ -0,0 +1 @@
+# See tests in +Card::StallableTest+
diff --git a/test/models/card/stallable_test.rb b/test/models/card/stallable_test.rb
new file mode 100644
index 000000000..05ee97d06
--- /dev/null
+++ b/test/models/card/stallable_test.rb
@@ -0,0 +1,36 @@
+require "test_helper"
+
+class Card::StallableTest < ActiveSupport::TestCase
+  setup do
+    Current.session = sessions(:david)
+  end
+
+  test "a card without activity spike is not stalled" do
+    assert_not cards(:logo).stalled?
+  end
+
+  test "a card with a recent activity spike is not stalled" do
+    cards(:logo).create_activity_spike!
+    assert_not cards(:logo).stalled?
+  end
+
+  test "a card with an old activity spike is stalled" do
+    cards(:logo).create_activity_spike!(updated_at: 3.months.ago)
+    assert cards(:logo).stalled?
+  end
+
+  # More fine-grained testing in Card::ActivitySpike::Detector
+  test "detect activity spikes" do
+    assert_not cards(:logo).stalled?
+
+    perform_enqueued_jobs only: Card::ActivitySpike::DetectionJob do
+      4.times do |index|
+        cards(:logo).comments.create(body: "Comment number #{index}")
+      end
+    end
+
+    travel_to 1.month.from_now
+
+    assert cards(:logo).reload.stalled?
+  end
+end