diff --git a/application/Espo/Core/Formula/Functions/DatetimeGroup/DayOfWeekType.php b/application/Espo/Core/Formula/Functions/DatetimeGroup/DayOfWeekType.php
index 7676de85b1..55191f5355 100644
--- a/application/Espo/Core/Formula/Functions/DatetimeGroup/DayOfWeekType.php
+++ b/application/Espo/Core/Formula/Functions/DatetimeGroup/DayOfWeekType.php
@@ -54,18 +54,15 @@ class DayOfWeekType extends \Espo\Core\Formula\Functions\Base
 
         $value = $this->evaluate($item->value[0]);
 
-        if (empty($value)) return 0;
+        if (empty($value)) return -1;
 
         if (strlen($value) > 11) {
-            $resultString = $this->getInjection('dateTime')->convertSystemDateTime($value, null, 'D');
+            $resultString = $this->getInjection('dateTime')->convertSystemDateTime($value, null, 'e');
         } else {
-            $resultString = $this->getInjection('dateTime')->convertSystemDate($value, 'D');
+            $resultString = $this->getInjection('dateTime')->convertSystemDate($value, 'e');
         }
 
-        $result = intval($resultString) + 1;
-        if ($result == 8) {
-            $result = 1;
-        }
+        $result = intval($resultString);
 
         return $result;
     }
diff --git a/tests/unit/Espo/Core/Formula/FormulaTest.php b/tests/unit/Espo/Core/Formula/FormulaTest.php
index eafe279c3c..82640671a9 100644
--- a/tests/unit/Espo/Core/Formula/FormulaTest.php
+++ b/tests/unit/Espo/Core/Formula/FormulaTest.php
@@ -1352,7 +1352,7 @@ class FormulaTest extends \PHPUnit_Framework_TestCase
             }
         ');
         $actual = $this->formula->process($item);
-        $this->assertEquals(6, $actual);
+        $this->assertEquals(5, $actual);
 
         $item = json_decode('
             {
@@ -1366,7 +1366,35 @@ class FormulaTest extends \PHPUnit_Framework_TestCase
             }
         ');
         $actual = $this->formula->process($item);
-        $this->assertEquals(1, $actual);
+        $this->assertEquals(0, $actual);
+
+        $item = json_decode('
+            {
+                "type": "datetime\\\\dayOfWeek",
+                "value": [
+                    {
+                        "type": "value",
+                        "value": ""
+                    }
+                ]
+            }
+        ');
+        $actual = $this->formula->process($item);
+        $this->assertEquals(-1, $actual);
+
+        $item = json_decode('
+            {
+                "type": "datetime\\\\dayOfWeek",
+                "value": [
+                    {
+                        "type": "value",
+                        "value": "2017-05-12"
+                    }
+                ]
+            }
+        ');
+        $actual = $this->formula->process($item);
+        $this->assertEquals(5, $actual);
     }
 
     function testDatetimeDiff()