Fix incorrect timestamp values when using fromString()

When using fromString() on DateTime objects that were in the global
timezone, an incorrect value would be returned.

Fixes #3743

lib/Cake/Test/Case/Utility/CakeTimeTest.php

@@ -846,21 +846,34 @@ public function testFromStringWithDateTime() {
 		$date = new DateTime('+1 hour', new DateTimeZone('America/New_York'));
 		$result = $this->Time->fromString($date, 'UTC');
 		$date->setTimezone(new DateTimeZone('UTC'));
-		$expected = $date->format('U') + $date->getOffset();
+		$expected = $date->getTimestamp() + $date->getOffset();
 
 		$this->assertWithinMargin($expected, $result, 1);
 
 		date_default_timezone_set('Australia/Melbourne');
 
 		$date = new DateTime('+1 hour', new DateTimeZone('America/New_York'));
 		$result = $this->Time->fromString($date, 'Asia/Kuwait');
+
 		$date->setTimezone(new DateTimeZone('Asia/Kuwait'));
 		$expected = $date->format('U') + $date->getOffset();
 		$this->assertWithinMargin($expected, $result, 1);
 
 		$this->_restoreSystemTimezone();
 	}
 
+/**
+ * Test that datetimes in the default timezone are not modified.
+ *
+ * @return void
+ */
+	public function testFromStringWithDateTimeNoConversion() {
+		Configure::write('Config.timezone', date_default_timezone_get());
+		$date = new DateTime('2013-04-09');
+		$result = $this->Time->fromString($date);
+		$this->assertEquals($result, $date->getTimestamp());
+	}
+
 /**
  * test converting time specifiers using a time definition localfe file
  *

lib/Cake/Utility/CakeTime.php

@@ -319,10 +319,15 @@ public static function fromString($dateString, $timezone = null) {
 
 		if (is_int($dateString) || is_numeric($dateString)) {
 			$date = intval($dateString);
-		} elseif (is_object($dateString) && $dateString instanceof DateTime) {
+		} elseif (
+			$dateString instanceof DateTime &&
+			$dateString->getTimezone()->getName() != date_default_timezone_get()
+		) {
 			$clone = clone $dateString;
 			$clone->setTimezone(new DateTimeZone(date_default_timezone_get()));
-			$date = (int)$clone->format('U') + $clone->getOffset();
+			$date = (int)$clone->getTimestamp() + $clone->getOffset();
+		} elseif ($dateString instanceof DateTime) {
+			$date = (int)$dateString->getTimeStamp();
 		} else {
 			$date = strtotime($dateString);
 		}