diff --git a/hc/api/models.py b/hc/api/models.py
index 52824ca0..aa5181ba 100644
--- a/hc/api/models.py
+++ b/hc/api/models.py
@@ -247,33 +247,40 @@ class Check(models.Model):
         ping.save()
 
     def outages_by_month(self, months=2):
-        now = timezone.now()
+        """ Calculate the number of outages and downtime minutes per month.
+
+        Returns a list of (datetime, downtime_in_secs, number_of_outages) tuples.
+
+        """
 
+        def monthkey(dt):
+            return dt.year, dt.month
+
+        # Will accumulate totals here.
+        # (year, month) -> [datetime, downtime_in_secs, number_of_outages]
         totals = {}
+        # Will collect flips and month boundaries here
         events = []
+
         for boundary in month_boundaries(months=months):
-            totals[(boundary.year, boundary.month)] = [boundary, 0, 0]
+            totals[monthkey(boundary)] = [boundary, 0, 0]
             events.append((boundary, "---"))
 
-        flips = self.flip_set.filter(created__gt=now - td(days=32 * months))
-        for flip in flips:
+        for flip in self.flip_set.filter(created__gt=boundary):
             events.append((flip.created, flip.old_status))
 
-        events.sort(reverse=True)
-
-        needle, status = now, self.status
-        for dt, old_status in events:
+        # Iterate through flips and month boundaries in reverse order,
+        # and for each "down" event increase the counters in `totals`.
+        dt, status = timezone.now(), self.status
+        for prev_dt, prev_status in sorted(events, reverse=True):
             if status == "down":
-                if (dt.year, dt.month) not in totals:
-                    break
-
-                delta = needle - dt
-                totals[(dt.year, dt.month)][1] += int(delta.total_seconds())
-                totals[(dt.year, dt.month)][2] += 1
+                delta = dt - prev_dt
+                totals[monthkey(prev_dt)][1] += int(delta.total_seconds())
+                totals[monthkey(prev_dt)][2] += 1
 
-            needle = dt
-            if old_status != "---":
-                status = old_status
+            dt = prev_dt
+            if prev_status != "---":
+                status = prev_status
 
         flattened = list(totals.values())
         flattened.sort(reverse=True)
diff --git a/hc/api/tests/test_check_model.py b/hc/api/tests/test_check_model.py
index fec533e5..3fea2eb8 100644
--- a/hc/api/tests/test_check_model.py
+++ b/hc/api/tests/test_check_model.py
@@ -1,8 +1,9 @@
 from datetime import datetime, timedelta
 
 from django.utils import timezone
-from hc.api.models import Check
+from hc.api.models import Check, Flip
 from hc.test import BaseTestCase
+from mock import patch
 
 
 class CheckModelTestCase(BaseTestCase):
@@ -164,3 +165,65 @@ class CheckModelTestCase(BaseTestCase):
 
         d = check.to_dict()
         self.assertEqual(d["next_ping"], "2000-01-01T01:00:00+00:00")
+
+    def test_outages_by_month_handles_no_flips(self):
+        check = Check.objects.create(project=self.project)
+        r = check.outages_by_month(10)
+        self.assertEqual(len(r), 10)
+        for dt, secs, outages in r:
+            self.assertEqual(secs, 0)
+            self.assertEqual(outages, 0)
+
+    def test_outages_by_month_handles_currently_down_check(self):
+        check = Check.objects.create(project=self.project, status="down")
+
+        r = check.outages_by_month(10)
+        self.assertEqual(len(r), 10)
+        for dt, secs, outages in r:
+            self.assertEqual(outages, 1)
+
+    @patch("hc.api.models.timezone.now")
+    def test_outages_by_month_handles_flip_one_day_ago(self, mock_now):
+        mock_now.return_value = datetime(2019, 7, 19, tzinfo=timezone.utc)
+
+        check = Check.objects.create(project=self.project, status="down")
+        flip = Flip(owner=check)
+        flip.created = datetime(2019, 7, 18, tzinfo=timezone.utc)
+        flip.old_status = "up"
+        flip.new_status = "down"
+        flip.save()
+
+        r = check.outages_by_month(10)
+        self.assertEqual(len(r), 10)
+        for dt, secs, outages in r:
+            if dt.month == 7:
+                self.assertEqual(secs, 86400)
+                self.assertEqual(outages, 1)
+            else:
+                self.assertEqual(secs, 0)
+                self.assertEqual(outages, 0)
+
+    @patch("hc.api.models.timezone.now")
+    def test_outages_by_month_handles_flip_two_months_ago(self, mock_now):
+        mock_now.return_value = datetime(2019, 7, 19, tzinfo=timezone.utc)
+
+        check = Check.objects.create(project=self.project, status="down")
+        flip = Flip(owner=check)
+        flip.created = datetime(2019, 5, 19, tzinfo=timezone.utc)
+        flip.old_status = "up"
+        flip.new_status = "down"
+        flip.save()
+
+        r = check.outages_by_month(10)
+        self.assertEqual(len(r), 10)
+        for dt, secs, outages in r:
+            if dt.month == 7:
+                self.assertEqual(outages, 1)
+            elif dt.month == 6:
+                self.assertEqual(secs, 30 * 86400)
+                self.assertEqual(outages, 1)
+            elif dt.month == 5:
+                self.assertEqual(outages, 1)
+            else:
+                self.assertEqual(secs, 0)
+                self.assertEqual(outages, 0)
diff --git a/templates/emails/summary-downtimes-html.html b/templates/emails/summary-downtimes-html.html
index d735d5d0..3b10b4cb 100644
--- a/templates/emails/summary-downtimes-html.html
+++ b/templates/emails/summary-downtimes-html.html
@@ -65,7 +65,7 @@
         {% for boundary, seconds, count in check.outages_by_month %}
         {% if count %}
             <td style="border-top: 1px solid #EDEFF2; padding: 16px 8px; font-family: Helvetica, Arial, sans-serif;">
-                {{ count }} outage{{ count|pluralize }}
+                {{ count }} outage{{ count|pluralize }},
                 <br />
                 {{ seconds|hc_approx_duration }} total
             </td>