diff --git a/analytics/lib/counts.py b/analytics/lib/counts.py
index d9550e74c0..9e3d2e26cf 100644
--- a/analytics/lib/counts.py
+++ b/analytics/lib/counts.py
@@ -375,6 +375,24 @@ count_message_by_stream_query = """
 """
 zerver_count_message_by_stream = ZerverCountQuery(Message, StreamCount, count_message_by_stream_query)
 
+check_useractivityinterval_by_user_query = """
+    INSERT INTO analytics_usercount
+        (user_id, realm_id, value, property, subgroup, end_time)
+    SELECT
+        zerver_userprofile.id, zerver_userprofile.realm_id, 1, '%(property)s', %(subgroup)s, %%(time_end)s
+    FROM zerver_userprofile
+    JOIN zerver_useractivityinterval
+    ON
+        zerver_userprofile.id = zerver_useractivityinterval.user_profile_id
+    WHERE
+        zerver_useractivityinterval.end >= %%(time_start)s AND
+        zerver_useractivityinterval.start < %%(time_end)s
+        %(join_args)s
+    GROUP BY zerver_userprofile.id %(group_by_clause)s
+"""
+zerver_check_useractivityinterval_by_user = ZerverCountQuery(
+    UserActivityInterval, UserCount, check_useractivityinterval_by_user_query)
+
 def do_pull_minutes_active(stat, start_time, end_time):
     # type: (CountStat, datetime, datetime) -> None
     timer_start = time.time()
@@ -410,6 +428,10 @@ count_stats_ = [
               (Message, 'sending_client_id'), CountStat.DAY),
     CountStat('messages_in_stream:is_bot:day', zerver_count_message_by_stream, {},
               (UserProfile, 'is_bot'), CountStat.DAY),
+    # The minutes=15 part is due to the 15 minutes added in
+    # zerver.lib.actions.do_update_user_activity_interval.
+    CountStat('15day_actives::day', zerver_check_useractivityinterval_by_user, {},
+              None, CountStat.DAY, interval=timedelta(days=15)-timedelta(minutes=15)),
     LoggingCountStat('active_users_log:is_bot:day', RealmCount, CountStat.DAY),
     CustomPullCountStat('minutes_active::day', UserCount, CountStat.DAY, do_pull_minutes_active)
 ]
diff --git a/analytics/tests/test_counts.py b/analytics/tests/test_counts.py
index 859d76006f..103b2965c5 100644
--- a/analytics/tests/test_counts.py
+++ b/analytics/tests/test_counts.py
@@ -489,6 +489,50 @@ class TestCountStats(AnalyticsTestCase):
             user_profile=user, start=self.TIME_ZERO-start_offset,
             end=self.TIME_ZERO-end_offset)
 
+    def test_15day_actives(self):
+        # type: () -> None
+        stat = COUNT_STATS['15day_actives::day']
+        self.current_property = stat.property
+
+        _15day = 15*self.DAY - 15*self.MINUTE
+
+        # Outside time range, should not appear. Also tests upper boundary.
+        user1 = self.create_user()
+        self.create_interval(user1, _15day + self.DAY, _15day + timedelta(seconds=1))
+        self.create_interval(user1, timedelta(0), -self.HOUR)
+
+        # On lower boundary, should appear
+        user2 = self.create_user()
+        self.create_interval(user2, _15day + self.DAY, _15day)
+
+        # Multiple intervals, including one outside boundary
+        user3 = self.create_user()
+        self.create_interval(user3, 20*self.DAY, 19*self.DAY)
+        self.create_interval(user3, 20*self.HOUR, 19*self.HOUR)
+        self.create_interval(user3, 20*self.MINUTE, 19*self.MINUTE)
+
+        # Intervals crossing boundary
+        user4 = self.create_user()
+        self.create_interval(user4, 20*self.DAY, 10*self.DAY)
+        user5 = self.create_user()
+        self.create_interval(user5, self.MINUTE, -self.MINUTE)
+
+        # Interval subsuming time range
+        user6 = self.create_user()
+        self.create_interval(user6, 20*self.DAY, -2*self.DAY)
+
+        # Second realm
+        user7 = self.create_user(realm=self.second_realm)
+        self.create_interval(user7, 20*self.MINUTE, 19*self.MINUTE)
+
+        do_fill_count_stat_at_hour(stat, self.TIME_ZERO)
+        self.assertTableState(UserCount, ['value', 'user'],
+                              [[1, user2], [1, user3], [1, user4], [1, user5], [1, user6], [1, user7]])
+        self.assertTableState(RealmCount, ['value', 'realm'],
+                              [[5, self.default_realm], [1, self.second_realm]])
+        self.assertTableState(InstallationCount, ['value'], [[6]])
+        self.assertTableState(StreamCount, [], [])
+
     def test_minutes_active(self):
         # type: () -> None
         stat = COUNT_STATS['minutes_active::day']
diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index 89fa65a7fc..c85fbcdf9f 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -2301,6 +2301,8 @@ def streams_to_dicts_sorted(streams):
 
 def do_update_user_activity_interval(user_profile, log_time):
     # type: (UserProfile, datetime.datetime) -> None
+    # Update any stats in analytics.lib.counts.count_stats_ that rely on
+    # this if the 15 minutes is changed to something else.
     effective_end = log_time + datetime.timedelta(minutes=15)
     # This code isn't perfect, because with various races we might end
     # up creating two overlapping intervals, but that shouldn't happen