diff --git a/Classes/PHPExcel/Calculation/DateTime.php b/Classes/PHPExcel/Calculation/DateTime.php
index 6eb96fd2..56c14079 100644
--- a/Classes/PHPExcel/Calculation/DateTime.php
+++ b/Classes/PHPExcel/Calculation/DateTime.php
@@ -1071,7 +1071,9 @@ class PHPExcel_Calculation_DateTime {
 	public static function DAYOFMONTH($dateValue = 1) {
 		$dateValue	= PHPExcel_Calculation_Functions::flattenSingleValue($dateValue);
 
-		if (is_string($dateValue = self::_getDateValue($dateValue))) {
+		if ($dateValue === null) {
+            $dateValue = 1;
+		} elseif (is_string($dateValue = self::_getDateValue($dateValue))) {
 			return PHPExcel_Calculation_Functions::VALUE();
 		} elseif ($dateValue == 0.0) {
 			return 0;
@@ -1114,7 +1116,9 @@ class PHPExcel_Calculation_DateTime {
 		}
 		$style = floor($style);
 
-		if (is_string($dateValue = self::_getDateValue($dateValue))) {
+		if ($dateValue === null) {
+            $dateValue = 1;
+		} elseif (is_string($dateValue = self::_getDateValue($dateValue))) {
 			return PHPExcel_Calculation_Functions::VALUE();
 		} elseif ($dateValue < 0.0) {
 			return PHPExcel_Calculation_Functions::NaN();
@@ -1180,7 +1184,9 @@ class PHPExcel_Calculation_DateTime {
 		}
 		$method = floor($method);
 
-		if (is_string($dateValue = self::_getDateValue($dateValue))) {
+		if ($dateValue === null) {
+            $dateValue = 1;
+		} elseif (is_string($dateValue = self::_getDateValue($dateValue))) {
 			return PHPExcel_Calculation_Functions::VALUE();
 		} elseif ($dateValue < 0.0) {
 			return PHPExcel_Calculation_Functions::NaN();
@@ -1216,7 +1222,9 @@ class PHPExcel_Calculation_DateTime {
 	public static function MONTHOFYEAR($dateValue = 1) {
 		$dateValue	= PHPExcel_Calculation_Functions::flattenSingleValue($dateValue);
 
-		if (is_string($dateValue = self::_getDateValue($dateValue))) {
+		if ($dateValue === null) {
+            $dateValue = 1;
+		} elseif (is_string($dateValue = self::_getDateValue($dateValue))) {
 			return PHPExcel_Calculation_Functions::VALUE();
 		} elseif ($dateValue < 0.0) {
 			return PHPExcel_Calculation_Functions::NaN();
@@ -1245,7 +1253,9 @@ class PHPExcel_Calculation_DateTime {
 	public static function YEAR($dateValue = 1) {
 		$dateValue	= PHPExcel_Calculation_Functions::flattenSingleValue($dateValue);
 
-		if (is_string($dateValue = self::_getDateValue($dateValue))) {
+		if ($dateValue === null) {
+            $dateValue = 1;
+		} elseif (is_string($dateValue = self::_getDateValue($dateValue))) {
 			return PHPExcel_Calculation_Functions::VALUE();
 		} elseif ($dateValue < 0.0) {
 			return PHPExcel_Calculation_Functions::NaN();