PHP date addition and subtraction calculation

<?php
    date_default_timezone_set(&#39;PRC&#39;); //默认时区
    echo "今天:",date("Y-m-d",time()),"\n";
    echo "今天:",date("Y-m-d",strtotime("18 june 2008")),"\n";
    echo "昨天:",date("Y-m-d",strtotime("-1 day")),"\n";
    echo "明天:",date("Y-m-d",strtotime("+1 day")),"\n";
    echo "一周后:",date("Y-m-d",strtotime("+1 week")),"\n";
    echo "一周零两天四小时两秒后:",date("Y-m-d G:H:s",strtotime("+1 week 2 days 4 hours 2 seconds")), "\n";
    echo "下个星期四:",date("Y-m-d",strtotime("next Thursday")),"\n";
    echo "上个周一:".date("Y-m-d",strtotime("last Monday"))."\n";
    echo "一个月前:".date("Y-m-d",strtotime("last month"))."\n";
    echo "一个月后:".date("Y-m-d",strtotime("+1 month"))."\n";
    echo "十年后:".date("Y-m-d",strtotime("+10 year"))."\n";
?>

<?php
    // 默认时区
    date_default_timezone_set(&#39;PRC&#39;);
     
    echo "今天: ", date(&#39;Y-m-d H:i:s&#39;), "\n";       // 输出当前时间
    echo "明天: ", date(&#39;Y-m-d H:i:s&#39;, strtotime(&#39;+1 day&#39;)), "\n";        // 输出明天时间
     
    //strtotime 可以接受第二个参数，类型timestamp为指定日期
    echo date(&#39;Y-m-d H:i:s&#39;, strtotime ("+1 day", strtotime(&#39;2017-11-11&#39;))), "\n";
     
     
    // 这里+1 day 可以修改参数1为任何想需要的数 
    // day也可以改成year（年）month（月）hour（小时）minute（分）second（秒）
    echo date(&#39;Y-m-d H:i:s&#39;, strtotime("+1 day +1 hour +1 minute")), "\n";
?>

<?php
    // 默认时区
    date_default_timezone_set(&#39;PRC&#39;);
 
    echo date("Y-m-d H:i:s"), "\n";
 
    // 日期天数相加函数
    echo date(&#39;Y-m-d&#39;, strtotime(&#39;+1 day&#39;, strtotime(&#39;2016-09-12&#39;))), "\n\n";
 
    echo strtotime(&#39;2016-09-12&#39;), "\n";
    echo date("Y-m-d", &#39;1473609600&#39;), "\n";
    echo date("Y-m-d", &#39;1573609600&#39;), "\n";
 
    // 日期天数相加函数
    $d = "2016-09-12 10:12:20";
    echo date("Y-m-d", strtotime("$d   +1   day")), "\n\n";  
 
    // 把日期转换成时间截
    function date2time($d){
        $year=((int)substr("$d",0,4));  //取得年份
        $month=((int)substr("$d",5,2)); //取得月份
        $day=((int)substr("$d",8,2));   //取得几号
        return mktime(0,0,0,$month,$day,$year);
    }
    echo date2time("2016-09-12"), "\n\n";
 
    $date_1="2066-09-12";
    $date_2="2016-09-12";
    $Date_List_a1=explode("-",$date_1);
    $Date_List_a2=explode("-",$date_2);
    $d1=mktime(0,0,0,$Date_List_a1[1],$Date_List_a1[2],$Date_List_a1[0]);
    $d2=mktime(0,0,0,$Date_List_a2[1],$Date_List_a2[2],$Date_List_a2[0]);
    $Days=round(($d1-$d2)/3600/24);
    echo "$date_2 -> $date_1 相差: $Days 天", "\n";
?>