diff --git a/src/PhpSpreadsheet/Calculation/DateTime.php b/src/PhpSpreadsheet/Calculation/DateTime.php
index 117f2779..0bd131c0 100644
--- a/src/PhpSpreadsheet/Calculation/DateTime.php
+++ b/src/PhpSpreadsheet/Calculation/DateTime.php
@@ -1137,10 +1137,14 @@ class DateTime
             $dateValue = 1;
         } elseif (is_string($dateValue = self::getDateValue($dateValue))) {
             return Functions::VALUE();
-        } elseif ($dateValue == 0.0) {
-            return 0;
-        } elseif ($dateValue < 0.0) {
-            return Functions::NAN();
+        }
+
+        if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_EXCEL) {
+            if ($dateValue < 0.0) {
+                return Functions::NAN();
+            } elseif ($dateValue < 1.0) {
+                return 0;
+            }
         }
 
         // Execute function
diff --git a/src/PhpSpreadsheet/Shared/Date.php b/src/PhpSpreadsheet/Shared/Date.php
index 0c13c0e1..b9f8426f 100644
--- a/src/PhpSpreadsheet/Shared/Date.php
+++ b/src/PhpSpreadsheet/Shared/Date.php
@@ -165,18 +165,23 @@ class Date
     public static function excelToDateTimeObject($excelTimestamp, $timeZone = null)
     {
         $timeZone = ($timeZone === null) ? self::getDefaultTimezone() : self::validateTimeZone($timeZone);
-        if ($excelTimestamp < 1.0) {
-            // Unix timestamp base date
-            $baseDate = new \DateTime('1970-01-01', $timeZone);
-        } else {
-            // MS Excel calendar base dates
-            if (self::$excelCalendar == self::CALENDAR_WINDOWS_1900) {
-                // Allow adjustment for 1900 Leap Year in MS Excel
-                $baseDate = ($excelTimestamp < 60) ? new \DateTime('1899-12-31', $timeZone) : new \DateTime('1899-12-30', $timeZone);
+        if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_EXCEL) {
+            if ($excelTimestamp < 1.0) {
+                // Unix timestamp base date
+                $baseDate = new \DateTime('1970-01-01', $timeZone);
             } else {
-                $baseDate = new \DateTime('1904-01-01', $timeZone);
+                // MS Excel calendar base dates
+                if (self::$excelCalendar == self::CALENDAR_WINDOWS_1900) {
+                    // Allow adjustment for 1900 Leap Year in MS Excel
+                    $baseDate = ($excelTimestamp < 60) ? new \DateTime('1899-12-31', $timeZone) : new \DateTime('1899-12-30', $timeZone);
+                } else {
+                    $baseDate = new \DateTime('1904-01-01', $timeZone);
+                }
             }
+        } else {
+            $baseDate = new \DateTime('1899-12-30', $timeZone);
         }
+
         $days = floor($excelTimestamp);
         $partDay = $excelTimestamp - $days;
         $hours = floor($partDay * 24);
@@ -185,7 +190,10 @@ class Date
         $partDay = $partDay * 60 - $minutes;
         $seconds = round($partDay * 60);
 
-        $interval = '+' . $days . ' days';
+        if ($days >= 0) {
+            $days = '+' . $days;
+        }
+        $interval = $days . ' days';
 
         return $baseDate->modify($interval)
             ->setTime($hours, $minutes, $seconds);
diff --git a/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php b/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
index 8959cb00..78ed021f 100644
--- a/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/DateTimeTest.php
@@ -174,12 +174,17 @@ class DateTimeTest extends PHPUnit_Framework_TestCase
     /**
      * @dataProvider providerDAY
      *
-     * @param mixed $expectedResult
+     * @param mixed $expectedResultExcel
+     * @param mixed $expectedResultOpenOffice
      */
-    public function testDAY($expectedResult, ...$args)
+    public function testDAY($expectedResultExcel, $expectedResultOpenOffice, ...$args)
     {
-        $result = DateTime::DAYOFMONTH(...$args);
-        self::assertEquals($expectedResult, $result, null, 1E-8);
+        $resultExcel = DateTime::DAYOFMONTH(...$args);
+        self::assertEquals($expectedResultExcel, $resultExcel, null, 1E-8);
+
+        Functions::setCompatibilityMode(Functions::COMPATIBILITY_OPENOFFICE);
+        $resultOpenOffice = DateTime::DAYOFMONTH(...$args);
+        self::assertEquals($expectedResultOpenOffice, $resultOpenOffice, null, 1E-8);
     }
 
     public function providerDAY()
diff --git a/tests/data/Calculation/DateTime/DAY.php b/tests/data/Calculation/DateTime/DAY.php
index 4bcd8e7a..affc4df3 100644
--- a/tests/data/Calculation/DateTime/DAY.php
+++ b/tests/data/Calculation/DateTime/DAY.php
@@ -4,31 +4,53 @@
 
 return [
     [
-        19,
+        19,         // Result for Excel
+        19,         // Result for OpenOffice
         22269,
     ],
     [
-        1,
+        1,          // Result for Excel
+        1,          // Result for OpenOffice
         30348,
     ],
     [
-        10,
+        10,         // Result for Excel
+        10,         // Result for OpenOffice
         30843,
     ],
     [
-        11,
+        11,         // Result for Excel
+        11,         // Result for OpenOffice
         '11-Nov-1918',
     ],
     [
-        28,
+        28,         // Result for Excel
+        28,         // Result for OpenOffice
         '28-Feb-1904',
     ],
     [
-        '#VALUE!',
+        '#VALUE!',  // Result for Excel
+        '#VALUE!',  // Result for OpenOffice
         'Invalid',
     ],
     [
-        '#NUM!',
+        '#NUM!',  // Result for Excel
+        29,       // Result for OpenOffice
         -1,
     ],
+    [
+        1,         // Result for Excel
+        31,        // Result for OpenOffice
+        1,
+    ],
+    [
+        0,         // Result for Excel
+        30,        // Result for OpenOffice
+        0.5,
+    ],
+    [
+        0,         // Result for Excel
+        30,        // Result for OpenOffice
+        0,
+    ],
 ];