diff --git a/CHANGELOG.md b/CHANGELOG.md
index 913f48a3..b113f74d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,9 @@ All notable changes to this project will be documented in this file.
 - Implement PagerDuty Simple Install Flow, remove PD Connect
 - Implement dark mode
 
+### Bug Fixes
+- Fix off-by-one-month error in monthly reports, downtime columns (#539)
+
 ## v1.20.0 - 2020-04-22
 
 ### Improvements
diff --git a/hc/api/models.py b/hc/api/models.py
index 7bc49db6..6b8dddff 100644
--- a/hc/api/models.py
+++ b/hc/api/models.py
@@ -307,7 +307,7 @@ class Check(models.Model):
         ping.exitstatus = exitstatus
         ping.save()
 
-    def downtimes(self, months=2):
+    def downtimes(self, months):
         """ Calculate the number of downtimes and downtime minutes per month.
 
         Returns a list of (datetime, downtime_in_secs, number_of_outages) tuples.
@@ -351,6 +351,11 @@ class Check(models.Model):
 
         return sorted(totals.values())
 
+    def past_downtimes(self):
+        """ Return downtime summary for two previous months. """
+
+        return self.downtimes(3)[:-1]
+
 
 class Ping(models.Model):
     id = models.BigAutoField(primary_key=True)
diff --git a/hc/lib/date.py b/hc/lib/date.py
index 76859c02..b4ea943e 100644
--- a/hc/lib/date.py
+++ b/hc/lib/date.py
@@ -1,5 +1,4 @@
 from datetime import datetime as dt
-from random import randint
 from django.utils import timezone
 
 
@@ -71,7 +70,7 @@ def format_approx_duration(td):
     return ""
 
 
-def month_boundaries(months=2):
+def month_boundaries(months):
     result = []
 
     now = timezone.now()
diff --git a/templates/emails/summary-downtimes-html.html b/templates/emails/summary-downtimes-html.html
index ae79d10c..b513d21e 100644
--- a/templates/emails/summary-downtimes-html.html
+++ b/templates/emails/summary-downtimes-html.html
@@ -60,7 +60,7 @@
                 </table>
             {% endif %}
         </td>
-        {% for boundary, seconds, count in check.downtimes %}
+        {% for boundary, seconds, count in check.past_downtimes %}
         {% if count %}
             <td style="border-top: 1px solid #EDEFF2; padding: 16px 8px; font-family: Helvetica, Arial, sans-serif;">
                 {{ count }}&nbsp;downtime{{ count|pluralize }},