echo date("Y-m-d H:i:s",strtotime("-1 day"));

例子

echo(strtotime("now"));

echo(strtotime("3 October 2005"));

echo(strtotime("+5 hours"));

echo(strtotime("+1 week"));

echo(strtotime("+1 week 3 days 7 hours 5 seconds"));

echo(strtotime("next Monday"));

echo(strtotime("last Sunday"));

?>输出：

1138614504

1128290400

1138632504

1139219304

1139503709

1139180400

1138489200

$time=time();

//一天的开始

$start=strtotime(date('Y-m-d',$time).("-8 hours").'00:00:01');

echo date('Y-m-d H:i:s',$start);

//一天的结束

$end=strtotime(date('Y-m-d',$time).("-8 hours").'23:59:59');

echo date('Y-m-d H:i:s',$end);

$collection_of_products = Core::getModel('checkout/salesordergift')->getCollection()

->addFieldToSelect('gift_type')

->addFieldToSelect('created_at')

->addFieldToFilter('gift_type','bag')

->addFieldToFilter('created_at',array('from'=>$day_start,'to'=>$day_end));