diff --git a/lib/statslib.php b/lib/statslib.php
index 9dfd7f05f74..a8970faa62e 100644
--- a/lib/statslib.php
+++ b/lib/statslib.php
@@ -987,18 +987,24 @@ function stats_get_base_daily($time=0) {
 function stats_get_base_weekly($time=0) {
     global $CFG;
 
-    $time = stats_get_base_daily($time);
+    $datetime = new DateTime();
+    $datetime->setTimestamp(stats_get_base_daily($time));
     $startday = $CFG->calendar_startwday;
 
     core_date::set_default_server_timezone();
     $thisday = date('w', $time);
 
+    $days = 0;
+
     if ($thisday > $startday) {
-        $time = $time - (($thisday - $startday) * 60*60*24);
+        $days = $thisday - $startday;
     } else if ($thisday < $startday) {
-        $time = $time - ((7 + $thisday - $startday) * 60*60*24);
+        $days = 7 + $thisday - $startday;
     }
-    return $time;
+
+    $datetime->sub(new DateInterval("P{$days}D"));
+
+    return $datetime->getTimestamp();
 }
 
 /**
diff --git a/lib/tests/statslib_test.php b/lib/tests/statslib_test.php
index f8ced493736..7af80e024f8 100644
--- a/lib/tests/statslib_test.php
+++ b/lib/tests/statslib_test.php
@@ -219,6 +219,34 @@ class core_statslib_testcase extends advanced_testcase {
         return $logfiles;
     }
 
+    /**
+     * Set of data for test_statlibs_get_base_weekly
+     *
+     * @return array Dates and timezones for which the first day of the week will be calculated
+     */
+    public function get_base_weekly_provider() {
+        return [
+            [
+                "startwday" => 0,
+                "timezone" => 'America/Chicago',
+                "timestart" => '18-03-2017 22:00',
+                "expected" => '12-03-2017 00:00:00'
+            ],
+            [
+                "startwday" => 0,
+                "timezone" => 'America/Chicago',
+                "date" => '25-03-2017 22:00',
+                "expected" => '19-03-2017 00:00:00'
+            ],
+            [
+                "startwday" => 1,
+                "timezone" => 'Atlantic/Canary',
+                "date" => '06-08-2018 22:00',
+                "expected" => '06-08-2018 00:00:00'
+            ],
+        ];
+    }
+
     /**
      * Compare the expected stats to those in the database.
      *
@@ -420,6 +448,27 @@ class core_statslib_testcase extends advanced_testcase {
         $this->assertEquals(24, ((1446526800 - 1446440400) / 60 ) / 60);
     }
 
+    /**
+     * Test the function that calculates the start of the week.
+     *
+     * @dataProvider get_base_weekly_provider
+     * @param int $startwday Day in which the week starts (Sunday = 0)
+     * @param string $timezone Default timezone
+     * @param string $timestart Date and time for which the first day of the week will be obtained
+     * @param string $expected Expected date of the first day of the week
+     */
+    public function test_statslib_get_base_weekly($startwday, $timezone, $timestart, $expected) {
+        $this->setTimezone($timezone);
+        $time = strtotime($timestart);
+        $expected = strtotime($expected);
+        set_config('calendar_startwday', $startwday);
+        set_config('statslastweekly', $time);
+
+        $weekstarttime = stats_get_base_weekly($time);
+
+        $this->assertEquals($expected, $weekstarttime);
+    }
+
     /**
      * Test the function that gets the action names.
      *