diff --git a/Tests/DataProvider/TestScalarDataProvider.php b/Tests/DataProvider/TestScalarDataProvider.php
index 1d15c78a..e5395bfa 100644
--- a/Tests/DataProvider/TestScalarDataProvider.php
+++ b/Tests/DataProvider/TestScalarDataProvider.php
@@ -104,7 +104,7 @@ public static function getIdTestData()
     public static function getDatetimeTestData()
     {
         return [
-            [null, null, false],
+            [null, null, true],
             [new \DateTime('now'), date('Y-m-d H:i:s'), true],
         ];
     }
@@ -112,7 +112,7 @@ public static function getDatetimeTestData()
     public static function getDatetimetzTestData()
     {
         return [
-            [null, null, false],
+            [null, null, true],
             [new \DateTime('now'), date('r'), true],
         ];
     }
@@ -120,7 +120,7 @@ public static function getDatetimetzTestData()
     public static function getDateTestData()
     {
         return [
-            [null, null, false],
+            [null, null, true],
             [new \DateTime('now'), date('Y-m-d'), true],
         ];
     }
@@ -130,7 +130,7 @@ public static function getTimestampTestData()
     {
         return [
             [new \DateTime('now'), time(), true],
-            [null, null, false],
+            [null, null, true],
         ];
     }
 
diff --git a/Tests/Issues/Issue90/Issue90Schema.php b/Tests/Issues/Issue90/Issue90Schema.php
new file mode 100644
index 00000000..03f0d28a
--- /dev/null
+++ b/Tests/Issues/Issue90/Issue90Schema.php
@@ -0,0 +1,59 @@
+<?php
+namespace Youshido\Tests\Issues\Issue90;
+
+use Youshido\GraphQL\Config\Schema\SchemaConfig;
+use Youshido\GraphQL\Schema\AbstractSchema;
+use Youshido\GraphQL\Type\Object\ObjectType;
+use Youshido\GraphQL\Type\Scalar\DateTimeType;
+
+class Issue90Schema extends AbstractSchema
+{
+
+    public function build(SchemaConfig $config)
+    {
+        $config->setQuery(
+            new ObjectType([
+                'name'   => 'QueryType',
+                'fields' => [
+                    'echo' => [
+                        'type'    => new DateTimeType('Y-m-d H:ia'),
+                        'args'    => [
+                            'date' => new DateTimeType('Y-m-d H:ia')
+                        ],
+                        'resolve' => function ($value, $args, $info) {
+
+                            if (isset($args['date'])) {
+                                return $args['date'];
+                            }
+
+                            return null;
+                        }
+                    ]
+                ]
+            ])
+        );
+
+        $config->setMutation(
+            new ObjectType([
+                'name'   => 'MutationType',
+                'fields' => [
+                    'echo' => [
+                        'type'    => new DateTimeType('Y-m-d H:ia'),
+                        'args'    => [
+                            'date' => new DateTimeType('Y-m-d H:ia')
+                        ],
+                        'resolve' => function ($value, $args, $info) {
+
+                            if (isset($args['date'])) {
+                                return $args['date'];
+                            }
+
+                            return null;
+                        }
+                    ]
+                ]
+            ])
+        );
+    }
+
+}
\ No newline at end of file
diff --git a/Tests/Issues/Issue90/Issue90Test.php b/Tests/Issues/Issue90/Issue90Test.php
new file mode 100644
index 00000000..e30b2729
--- /dev/null
+++ b/Tests/Issues/Issue90/Issue90Test.php
@@ -0,0 +1,76 @@
+<?php
+
+namespace Youshido\Tests\Issues\Issue90;
+
+use Youshido\GraphQL\Execution\Processor;
+use Youshido\Tests\Issues\Issue90\Issue90Schema;
+
+/**
+ * User: stefano.corallo
+ * Date: 25/11/16
+ * Time: 9.39
+ */
+class Issue90Test extends \PHPUnit_Framework_TestCase
+{
+
+    public function testQueryDateTimeTypeWithDateParameter()
+    {
+        $schema = new Issue90Schema();
+        $processor = new Processor($schema);
+        $processor->processPayload("query{ echo(date: \"2016-11-25 09:53am\") }");
+        $res = $processor->getResponseData();
+
+        self::assertCount(1, $res, "Invalid response array received"); //only data expected
+
+        self::assertNotNull($res['data']['echo'], "Invalid echo response received");
+
+        self::assertEquals("2016-11-25 09:53am", $res['data']['echo']);
+    }
+
+    public function testQueryDateTimeTypeWithoutParameter()
+    {
+        $processor = new Processor(new Issue90Schema());
+        $processor->processPayload("query{ echo }");
+        $res = $processor->getResponseData();
+
+        self::assertCount(1, $res, "Invalid response array received"); //only data expected
+
+        self::assertNull($res['data']['echo']);
+    }
+
+    public function testQueryDateTimeTypeWithNullParameter()
+    {
+        $processor = new Processor(new Issue90Schema());
+        $processor->processPayload("query{ echo(date: null) }");
+        $res = $processor->getResponseData();
+
+        self::assertCount(1, $res, "Invalid response array received"); //only data expected
+
+        self::assertNull($res['data']['echo'], "Error Quering with explicit date null parameter ");
+    }
+
+    public function testMutatingDateTimeWithParameter()
+    {
+        $schema = new Issue90Schema();
+        $processor = new Processor($schema);
+        $processor->processPayload("mutation{ echo(date: \"2016-11-25 09:53am\") }");
+        $res = $processor->getResponseData();
+
+        self::assertCount(1, $res, "Invalid response array received"); //only data expected
+
+        self::assertNotNull($res['data']['echo'], "Invalid echo response received during mutation of date parameter");
+
+        self::assertEquals("2016-11-25 09:53am", $res['data']['echo']);
+    }
+
+    public function testMutatingDateTimeWithExplicitNullParameter()
+    {
+        $schema = new Issue90Schema();
+        $processor = new Processor($schema);
+        $processor->processPayload("mutation{ echo(date: null) }");
+        $res = $processor->getResponseData();
+
+        self::assertCount(1, $res, "Invalid response array received"); //only data expected
+        self::assertNull($res['data']['echo'], "Invalid echo response received during mutation of date parameter with explicit null value");
+    }
+}
\ No newline at end of file
diff --git a/Tests/Library/Type/EnumTypeTest.php b/Tests/Library/Type/EnumTypeTest.php
index c6f356ac..c6969712 100644
--- a/Tests/Library/Type/EnumTypeTest.php
+++ b/Tests/Library/Type/EnumTypeTest.php
@@ -120,7 +120,7 @@ public function testNormalCreatingParams()
         $this->assertEquals($enumType->getNamedType(), $enumType);
 
         $this->assertFalse($enumType->isValidValue($enumType));
-        $this->assertFalse($enumType->isValidValue(null));
+        $this->assertTrue($enumType->isValidValue(null));
 
         $this->assertTrue($enumType->isValidValue(true));
         $this->assertTrue($enumType->isValidValue('disable'));
diff --git a/Tests/Library/Type/ScalarTypeTest.php b/Tests/Library/Type/ScalarTypeTest.php
index f6070c8c..73608ea8 100644
--- a/Tests/Library/Type/ScalarTypeTest.php
+++ b/Tests/Library/Type/ScalarTypeTest.php
@@ -36,7 +36,7 @@ public function testScalarPrimitives()
             foreach (call_user_func(['Youshido\Tests\DataProvider\TestScalarDataProvider', $testDataMethod]) as list($data, $serialized, $isValid)) {
 
                 $this->assertSerialization($scalarType, $data, $serialized);
-                $this->assertParse($scalarType, $data, $serialized);
+                $this->assertParse($scalarType, $data, $serialized, $typeName);
 
                 if ($isValid) {
                     $this->assertTrue($scalarType->isValidValue($data), $typeName . ' validation for :' . serialize($data));
@@ -65,9 +65,13 @@ private function assertSerialization(AbstractScalarType $object, $input, $expect
         $this->assertEquals($expected, $object->serialize($input), $object->getName() . ' serialize for: ' . serialize($input));
     }
 
-    private function assertParse(AbstractScalarType $object, $input, $expected)
+    private function assertParse(AbstractScalarType $object, $input, $expected, $typeName)
     {
-        $this->assertEquals($expected, $object->parseValue($input), $object->getName() . ' serialize for: ' . serialize($input));
+        $parsed = $object->parseValue($input);
+        if ($parsed instanceof \DateTime) {
+            $expected = \DateTime::createFromFormat($typeName == 'datetime' ? 'Y-m-d H:i:s' : 'D, d M Y H:i:s O', $expected);
+        }
+        $this->assertEquals($expected, $parsed, $object->getName() . ' parse for: ' . serialize($input));
     }
 
 }
diff --git a/Tests/Schema/InputObjectDefaultValuesTest.php b/Tests/Schema/InputObjectDefaultValuesTest.php
index decf1fd2..491a69cf 100644
--- a/Tests/Schema/InputObjectDefaultValuesTest.php
+++ b/Tests/Schema/InputObjectDefaultValuesTest.php
@@ -22,12 +22,12 @@ public function testDefaultEnum()
             'name'   => 'InternalStatus',
             'values' => [
                 [
-                    'name'  => 1,
-                    'value' => 'ACTIVE'
+                    'name'  => 'ACTIVE',
+                    'value' => 1,
                 ],
                 [
-                    'name'  => 0,
-                    'value' => 'DISABLED'
+                    'name'  => 'DISABLED',
+                    'value' => 0,
                 ],
             ]
         ]);
@@ -36,8 +36,8 @@ public function testDefaultEnum()
                 'name'   => 'RootQuery',
                 'fields' => [
                     'stringQuery' => [
-                        'type'    => new StringType(),
-                        'args'    => [
+                        'type'       => new StringType(),
+                        'args'       => [
                             'statObject' => new InputObjectType([
                                 'name'   => 'StatObjectType',
                                 'fields' => [
@@ -49,12 +49,26 @@ public function testDefaultEnum()
                                 ]
                             ])
                         ],
-                        'resolve' => function ($source, $args) {
+                        'resolve'    => function ($source, $args) {
                             return sprintf('Result with level %s and status %s',
                                 $args['statObject']['level'], $args['statObject']['status']
                             );
                         },
                     ],
+                    'enumObject' => [
+                        'type' => new ObjectType([
+                            'name'   => 'EnumObject',
+                            'fields' => [
+                                'status' => $enumType
+                            ]
+                        ]),
+                        'resolve' => function() {
+                            return [
+                                'status' => null
+                            ];
+                        }
+                    ],
+
                 ]
             ])
         ]);
@@ -62,9 +76,24 @@ public function testDefaultEnum()
         $processor = new Processor($schema);
         $processor->processPayload('{ stringQuery(statObject: { level: 1 }) }');
         $result = $processor->getResponseData();
+        $this->assertEquals(['data' => [
+            'stringQuery' => 'Result with level 1 and status 1'
+        ]], $result);
+
+        $processor->processPayload('{ stringQuery(statObject: { level: 1, status: DISABLED }) }');
+        $result = $processor->getResponseData();
 
         $this->assertEquals(['data' => [
-            'stringQuery' => 'Result with level 1 and status ACTIVE'
+            'stringQuery' => 'Result with level 1 and status 0'
+        ]], $result);
+
+        $processor->processPayload('{ enumObject { status } }');
+        $result = $processor->getResponseData();
+
+        $this->assertEquals(['data' => [
+            'enumObject' => [
+                'status' => null
+            ]
         ]], $result);
     }
 
diff --git a/Tests/Schema/InputParseTest.php b/Tests/Schema/InputParseTest.php
index 57bc0be9..d3bc3b86 100644
--- a/Tests/Schema/InputParseTest.php
+++ b/Tests/Schema/InputParseTest.php
@@ -27,13 +27,13 @@ public function testDateInput($query, $expected)
                     'stringQuery' => [
                         'type'    => new StringType(),
                         'args'    => [
-                            'from'   => new DateTimeType(),
+                            'from'   => new DateTimeType('Y-m-d H:i:s'),
                             'fromtz' => new DateTimeTzType(),
                         ],
                         'resolve' => function ($source, $args) {
                             return sprintf('Result with %s date and %s tz',
-                                empty($args['from']) ? 'default' : $args['from'],
-                                empty($args['fromtz']) ? 'default' : $args['fromtz']
+                                empty($args['from']) ? 'default' : $args['from']->format('Y-m-d H:i:s'),
+                                empty($args['fromtz']) ? 'default' : $args['fromtz']->format('r')
                             );
                         },
                     ],
diff --git a/examples/js/index.js b/examples/js/index.js
index 1ad22ddd..257147f0 100644
--- a/examples/js/index.js
+++ b/examples/js/index.js
@@ -180,6 +180,12 @@ const blogSchema = new GraphQLSchema({
                     return [DataProvider.getPost(2), DataProvider.getBanner(3)];
                 }
             },
+            enumNull: {
+                type: postStatus,
+                resolve: () => {
+                    return null;
+                }
+            },
             scalarList: {
                 type: new GraphQLList(new GraphQLObjectType({
                     name: 'scalarObject',
@@ -224,7 +230,7 @@ const blogSchema = new GraphQLSchema({
     })
 });
 
-var query = '{ scalarList(count: 12) { id, cost } }';
+var query = '{ enumNull }';
 graphql(blogSchema, query).then(result => {
     console.log(JSON.stringify(result, null, 5));
     process.exit(0);
diff --git a/src/Execution/Processor.php b/src/Execution/Processor.php
index 6c242614..4ed131f5 100644
--- a/src/Execution/Processor.php
+++ b/src/Execution/Processor.php
@@ -249,7 +249,7 @@ private function prepareArgumentValue($argumentValue, AbstractType $argumentType
                     foreach($argumentType->getFields() as $field) {
                         /** @var $field Field */
                         if ($field->getConfig()->has('default')) {
-                            $result[$field->getName()] = $field->getConfig()->get('default');
+                            $result[$field->getName()] = $field->getType()->getNullableType()->parseInputValue($field->getConfig()->get('default'));
                         }
                     }
                     foreach ($argumentValue->getValue() as $key => $item) {
diff --git a/src/Field/AbstractField.php b/src/Field/AbstractField.php
index 51c36177..44a68d1f 100644
--- a/src/Field/AbstractField.php
+++ b/src/Field/AbstractField.php
@@ -26,7 +26,6 @@ abstract class AbstractField implements FieldInterface
     }
     protected $isFinal = false;
 
-    private $resolveFunctionCache = null;
     private $nameCache            = null;
 
     public function __construct(array $config = [])
diff --git a/src/Type/AbstractType.php b/src/Type/AbstractType.php
index 352461e4..95596653 100644
--- a/src/Type/AbstractType.php
+++ b/src/Type/AbstractType.php
@@ -53,6 +53,11 @@ public function parseValue($value)
         return $value;
     }
 
+    public function parseInputValue($value)
+    {
+        return $this->parseValue($value);
+    }
+
     public function serialize($value)
     {
         return $value;
diff --git a/src/Type/Enum/AbstractEnumType.php b/src/Type/Enum/AbstractEnumType.php
index 199c82f0..80a33155 100644
--- a/src/Type/Enum/AbstractEnumType.php
+++ b/src/Type/Enum/AbstractEnumType.php
@@ -48,6 +48,7 @@ public function getKind()
      */
     public function isValidValue($value)
     {
+        if (is_null($value)) return true;
         foreach ($this->getConfig()->get('values') as $item) {
             if ($value === $item['name'] || $value === $item['value']) {
                 return true;
@@ -84,4 +85,15 @@ public function parseValue($value)
         return null;
     }
 
+    public function parseInputValue($value)
+    {
+        foreach ($this->getConfig()->get('values') as $valueItem) {
+            if ($value === $valueItem['value']) {
+                return $valueItem['name'];
+            }
+        }
+
+        return null;
+    }
+
 }
diff --git a/src/Type/Scalar/DateTimeType.php b/src/Type/Scalar/DateTimeType.php
index 49d27d43..692b806e 100644
--- a/src/Type/Scalar/DateTimeType.php
+++ b/src/Type/Scalar/DateTimeType.php
@@ -23,33 +23,48 @@ public function getName()
         return 'DateTime';
     }
 
-    /**
-     * @param $value \DateTime
-     * @return null|string
-     */
-    public function serialize($value)
+    public function isValidValue($value)
     {
-        if ($value === null) {
-            return null;
+        if ((is_object($value) && $value instanceof \DateTime) || is_null($value)) {
+            return true;
+        } else if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } else {
+            $date = null;
         }
 
-        return $value->format($this->format);
+        return $date ? true : false;
     }
 
-    public function parseValue($value)
+    public function serialize($value)
     {
-        return is_object($value) ? $this->serialize($value) : $value;
+        $date = null;
+
+        if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } elseif ($value instanceof \DateTime) {
+            $date = $value;
+        }
+
+        return $date ? $date->format($this->format) : null;
     }
 
-    public function isValidValue($value)
+    public function parseValue($value)
     {
-        if (is_object($value)) {
-            return true;
+        if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } elseif ($value instanceof \DateTime) {
+            $date = $value;
+        } else {
+            $date = false;
         }
 
-        $d = \DateTime::createFromFormat($this->format, $value);
+        return $date ?: null;
+    }
 
-        return $d && $d->format($this->format) == $value;
+    private function createFromFormat($value)
+    {
+        return \DateTime::createFromFormat($this->format, $value);
     }
 
     public function getDescription()
diff --git a/src/Type/Scalar/DateTimeTzType.php b/src/Type/Scalar/DateTimeTzType.php
index f3bbd375..cd1a6db4 100644
--- a/src/Type/Scalar/DateTimeTzType.php
+++ b/src/Type/Scalar/DateTimeTzType.php
@@ -10,38 +10,54 @@
 
 class DateTimeTzType extends AbstractScalarType
 {
+    private $format = 'D, d M Y H:i:s O';
+
     public function getName()
     {
         return 'DateTimeTz';
     }
-
-    /**
-     * @param $value \DateTime
-     * @return null|string
-     */
-    public function serialize($value)
+    public function isValidValue($value)
     {
-        if ($value === null) {
-            return null;
+        if ((is_object($value) && $value instanceof \DateTime) || is_null($value)) {
+            return true;
+        } else if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } else {
+            $date = null;
         }
 
-        return $value->format('r');
+        return $date ? true : false;
     }
 
-    public function parseValue($value)
+    public function serialize($value)
     {
-        return is_object($value) ? $this->serialize($value) : $value;
+        $date = null;
+
+        if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } elseif ($value instanceof \DateTime) {
+            $date = $value;
+        }
+
+        return $date ? $date->format($this->format) : null;
     }
 
-    public function isValidValue($value)
+    public function parseValue($value)
     {
-        if (is_object($value)) {
-            return true;
+        if (is_string($value)) {
+            $date = $this->createFromFormat($value);
+        } elseif ($value instanceof \DateTime) {
+            $date = $value;
+        } else {
+            $date = false;
         }
 
-        $d = \DateTime::createFromFormat('D, d M Y H:i:s O', $value);
+        return $date ?: null;
+    }
 
-        return $d && $d->format('r') == $value;
+    private function createFromFormat($value)
+    {
+        return \DateTime::createFromFormat($this->format, $value);
     }
 
     public function getDescription()
diff --git a/src/Type/Scalar/DateType.php b/src/Type/Scalar/DateType.php
index 007d6a8f..b37c92a5 100644
--- a/src/Type/Scalar/DateType.php
+++ b/src/Type/Scalar/DateType.php
@@ -37,7 +37,7 @@ public function serialize($value)
 
     public function isValidValue($value)
     {
-        if (is_object($value)) {
+        if (is_null($value) || is_object($value)) {
             return true;
         }
 
diff --git a/src/Type/Scalar/TimestampType.php b/src/Type/Scalar/TimestampType.php
index 6923ed48..06a57e28 100644
--- a/src/Type/Scalar/TimestampType.php
+++ b/src/Type/Scalar/TimestampType.php
@@ -32,7 +32,7 @@ public function serialize($value)
 
     public function isValidValue($value)
     {
-        if (is_object($value)) {
+        if (is_null($value) || is_object($value)) {
             return true;
         }
 
diff --git a/src/Type/TypeFactory.php b/src/Type/TypeFactory.php
index 5e05fecf..25f2550b 100644
--- a/src/Type/TypeFactory.php
+++ b/src/Type/TypeFactory.php
@@ -26,11 +26,7 @@ public static function getScalarType($type)
     {
         if (TypeService::isScalarType($type)) {
             if (is_object($type)) {
-                $typeName = $type->getName();
-                if (empty(self::$objectsHash[$typeName])) {
-                    self::$objectsHash[$typeName] = $type;
-                }
-                return self::$objectsHash[$typeName];
+                return $type;
             }
             if (empty(self::$objectsHash[$type])) {
                 $name = ucfirst($type);