PHP で日付と時刻のクラス Carbon を使用する方法

use Carbon\Carbon;
$today = new Carbon('2018-08-01');

$first_day = new Carbon('first day of this month');
$first_day = new Carbon('first day of last month');
return $month->format('Y-m');   // 2018-08

echo Carbon::today()->toDateString();// 2018-09-10

Carbon::yesterday()->toDateString();

Carbon::now();             // 2018-10-08 14:24:15.074542 Asia/Shanghai (+08:00)
echo Carbon::now();    // 2018-10-08 14:28:10

$start  = new Carbon('2018-10-04 15:00:03');
$end    = new Carbon('2018-10-05 17:00:09');
$start->diff($end)->format('%H:%I:%S');
02:00:06

>>> Carbon::parse('today 8am');
=> Carbon\Carbon @1563148800 {#3955
     date: 2019-07-15 08:00:00.0 Asia/Shanghai (+08:00),
   }
>>> Carbon::parse('yesterday 6pm');
=> Carbon\Carbon @1563098400 {#3954
     date: 2019-07-14 18:00:00.0 Asia/Shanghai (+08:00),
   }

$orders = Order::whereDate('created_at', Carbon::today())->get();

date("Ymd", strtotime("+7 day"));    // "20190716"
date("Ymd", strtotime("-7 day"));