diff --git a/src/PhpSpreadsheet/Calculation/DateTime.php b/src/PhpSpreadsheet/Calculation/DateTime.php
index ceda521c..9f7d32f3 100644
--- a/src/PhpSpreadsheet/Calculation/DateTime.php
+++ b/src/PhpSpreadsheet/Calculation/DateTime.php
@@ -1242,10 +1242,14 @@ class DateTime
         $PHPDateObject = \PhpOffice\PhpSpreadsheet\Shared\Date::excelToDateTimeObject($dateValue);
         $dayOfYear = $PHPDateObject->format('z');
         $PHPDateObject->modify('-' . $dayOfYear . ' days');
-        $dow = $PHPDateObject->format('w');
-        $daysInFirstWeek = 7 - (($dow + (2 - $method)) % 7);
-        $dayOfYear -= $daysInFirstWeek;
-        $weekOfYear = ceil($dayOfYear / 7) + 1;
+        $firstDayOfFirstWeek = $PHPDateObject->format('w');
+        $daysInFirstWeek = (6 - $firstDayOfFirstWeek + $method) % 7;
+        $interval = $dayOfYear - $daysInFirstWeek;
+        $weekOfYear = floor($interval / 7) + 1;
+
+        if ($daysInFirstWeek) {
+            ++$weekOfYear;
+        }
 
         return (int) $weekOfYear;
     }
diff --git a/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php b/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
index a7e8c3c8..d33a74ae 100644
--- a/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
@@ -140,12 +140,9 @@ class DateTimeTest extends \PHPUnit_Framework_TestCase
 
     /**
      * @dataProvider providerWEEKNUM
-     * @group fail19
      */
     public function testWEEKNUM()
     {
-        $this->markTestIncomplete('TODO: This test should be fixed');
-
         $args = func_get_args();
         $expectedResult = array_pop($args);
         $result = call_user_func_array([DateTime::class, 'WEEKOFYEAR'], $args);
diff --git a/tests/data/Calculation/DateTime/WEEKNUM.php b/tests/data/Calculation/DateTime/WEEKNUM.php
index bdfa7d4a..494ef8be 100644
--- a/tests/data/Calculation/DateTime/WEEKNUM.php
+++ b/tests/data/Calculation/DateTime/WEEKNUM.php
@@ -6,14 +6,54 @@ return [
         1,
         52,
     ],
+    [
+        '2000-01-01',
+        1,
+        1,
+    ],
+    [
+        '2000-01-02',
+        1,
+        2,
+    ],
+    [
+        '2000-01-01',
+        2,
+        1,
+    ],
+    [
+        '2000-01-03',
+        2,
+        2,
+    ],
     [
         '1995-01-01',
         1,
         1,
     ],
+    [
+        '1995-01-07',
+        1,
+        1,
+    ],
+    [
+        '1995-01-08',
+        1,
+        2,
+    ],
+    [
+        '1995-01-01',
+        2,
+        1,
+    ],
+    [
+        '1995-01-02',
+        2,
+        2,
+    ],
     [
         '3/7/1977',
-        27,
+        28,
     ],
     [
         '3/7/1977',