diff --git a/src/Chronos.php b/src/Chronos.php
index 6d8e3029..409a2e7f 100644
--- a/src/Chronos.php
+++ b/src/Chronos.php
@@ -96,7 +96,7 @@ public function __construct($time = null, $tz = null)
         }
 
         if ($tz !== $testInstance->getTimezone()) {
-            $testInstance = $testInstance->setTimezone($tz);
+            $testInstance = $testInstance->setTimezone($tz === null ? date_default_timezone_get() : $tz);
         }
 
         $time = $testInstance->format('Y-m-d H:i:s.u');
diff --git a/src/Date.php b/src/Date.php
index 9a7ff362..aa20414c 100644
--- a/src/Date.php
+++ b/src/Date.php
@@ -73,7 +73,7 @@ public function __construct($time = null, $tz = null)
         }
 
         if ($tz !== $testInstance->getTimezone()) {
-            $testInstance = $testInstance->setTimezone($tz);
+            $testInstance = $testInstance->setTimezone($tz === null ? date_default_timezone_get() : $tz);
         }
 
         $time = $testInstance->format('Y-m-d 00:00:00');
diff --git a/src/MutableDate.php b/src/MutableDate.php
index bf7b673d..9b5d5b74 100644
--- a/src/MutableDate.php
+++ b/src/MutableDate.php
@@ -68,14 +68,14 @@ public function __construct($time = null, $tz = null)
             return parent::__construct($time, $tz);
         }
 
-        $testInstance = static::getTestNow();
+        $testInstance = clone static::getTestNow();
         if ($relative) {
-            $testInstance = clone $testInstance;
+            $testInstance = $testInstance;
             $testInstance = $testInstance->modify($time);
         }
 
         if ($tz !== $testInstance->getTimezone()) {
-            $testInstance = $testInstance->setTimezone($tz);
+            $testInstance = $testInstance->setTimezone($tz === null ? date_default_timezone_get() : $tz);
         }
 
         $time = $testInstance->format('Y-m-d 00:00:00');
diff --git a/src/MutableDateTime.php b/src/MutableDateTime.php
index 4f5b507c..4f1767cc 100644
--- a/src/MutableDateTime.php
+++ b/src/MutableDateTime.php
@@ -65,14 +65,14 @@ public function __construct($time = null, $tz = null)
             return parent::__construct($time, $tz);
         }
 
-        $testInstance = static::getTestNow();
+        $testInstance = clone static::getTestNow();
         if ($relative) {
-            $testInstance = clone $testInstance;
+            $testInstance = $testInstance;
             $testInstance = $testInstance->modify($time);
         }
 
         if ($tz !== $testInstance->getTimezone()) {
-            $testInstance = $testInstance->setTimezone($tz);
+            $testInstance = $testInstance->setTimezone($tz === null ? date_default_timezone_get() : $tz);
         }
 
         $time = $testInstance->format('Y-m-d H:i:s.u');
diff --git a/tests/DateTime/TestingAidsTest.php b/tests/DateTime/TestingAidsTest.php
index eaa83c7e..28224f17 100644
--- a/tests/DateTime/TestingAidsTest.php
+++ b/tests/DateTime/TestingAidsTest.php
@@ -14,6 +14,7 @@
 namespace Cake\Chronos\Test\DateTime;
 
 use TestCase;
+use DateTimeZone;
 
 class TestingAidsTest extends TestCase
 {
@@ -158,4 +159,19 @@ public function testTimeZoneWithTestValueSet($class)
         $this->assertSame('2013-07-01T11:00:00-05:00', $class::parse('now', 'America/Mexico_City')->toIso8601String());
         $this->assertSame('2013-07-01T09:00:00-07:00', $class::parse('now', 'America/Vancouver')->toIso8601String());
     }
+
+    /**
+     * @dataProvider classNameProvider
+     * @return void
+     */
+    public function testNullTimezone($class)
+    {
+       $c = new $class('2016-01-01 00:00:00', 'Europe/Copenhagen');
+       $class::setTestNow($c);
+
+       $result = new $class('now', null);
+       $this->assertEquals(new DateTimeZone('America/Toronto'), $result->tz);
+       $this->assertEquals('2015-12-31 18:00:00', $result->format('Y-m-d H:i:s'));
+       $this->assertEquals(new DateTimeZone('Europe/Copenhagen'), $class::getTestNow()->tz);
+    }
 }
diff --git a/tests/TestCase.php b/tests/TestCase.php
index 7e39b127..e49fb8a4 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -33,6 +33,10 @@ protected function setUp()
     protected function tearDown()
     {
         date_default_timezone_set($this->saveTz);
+        MutableDateTime::setTestNow(null);
+        Chronos::setTestNow(null);
+        MutableDate::setTestNow(null);
+        Date::setTestNow(null);
     }
 
     public function classNameProvider()