$datetime = new DateTime;

$datetime = new \DateTime;

 $datetime = new \DateTime;
 print_r($datetime->format('Y-m-d H:i:s'));

$datetime = new \DateTime('2016-06-13');
print_r($datetime);

$datetime = \DateTime::createFromFormat('Ymd', '20160618');
print_r($datetime->format('Y-m-d'));

//方法1(php5.2)：
$datetime = new \DateTime();
echo $datetime->format('U');exit;

//方法2(php5.3)推荐
$datetime = new \DateTime();
echo $datetime->getTimestamp();exit;

$datetime = new \DateTime();
$datetime->setTimestamp(1465783744);
echo $datetime->format('Y-m-d H:i:s');

$datetime1 = new \DateTime('2016-01-01 10:11:18');
$datetime2 = new \DateTime('2017-05-11 22:21:21');
$interval = $datetime1->diff($datetime2);
print_r($interval->format('%Y'));
//%表示使用格式化，R表示是大于这个日期(+)，还是小于这个日期（-）,
//a表示大于或小于多少天，时分秒正常使用y,m,d,h,i,s

$datetime = new \DateTime();
$interval = new \DateInterval('P2DT5H');
//或者使用createFromDateString方法
//$interval = \DateInterval::createFromDateString('1 month');
//修改DateTime实例
$datetime->add($interval);
echo $datetime->format('Y-m-d H:i:s');

$datetime = new \DateTime();
$interval = new \DateInterval('P2DT5H');
$datetime->sub($interval);
echo $datetime->format('Y-m-d H:i:s');
//ps:有个modify方法，这个方法是减去30，并不是像前推1天，输出的还是12月
$datetime = new \DateTime('2014/12/31');
$datetime->modify( '-1 month' );
print_r($datetime);exit;

$datetime = new \DateTime();
$datetime->setDate(2015, 2, 28);
echo $datetime->format('Y-m-d');exit;

$datetime = new \DateTime();
$datetime->setTime(20, 20, 24);
echo $datetime->format('Y-m-d H:i:s');exit;

$timezone = new \DateTimeZone('Asia/Calcutta');
$datetime = new \DateTime();
$datetime->setTimezone($timezone);
print_r($datetime->format('Y-m-d H:i:s'));exit;

 $date = new \DateTime(null, new DateTimeZone('Asia/Shanghai'));
 $tz = $date->getTimezone();
 echo $tz->getName();

$dateTimeZoneTaipei = new \DateTimeZone("Asia/Taipei");
$dateTimeZoneJapan = new \DateTimeZone("Asia/Tokyo");
$dateTimeTaipei = new \DateTime("now", $dateTimeZoneTaipei);
$dateTimeJapan = new \DateTime("now", $dateTimeZoneJapan);
$timeOffset = $dateTimeZoneJapan->getOffset($dateTimeTaipei);
print_r($timeOffset);exit;

$interval = new \DateInterval('P2Y4DT6H8M');
echo $interval->format('%d days');

$datetime = new \DateTime();
$interval = \DateInterval::createFromDateString('-1 day');
$period = new \DatePeriod($datetime, $interval, 3);
foreach ($period as $date) {
 echo $date->format('Y-m-d'), PHP_EOL;
}