With a \ before a Class declaration you are calling the root namespace:
$now = \Carbon\Carbon::now()->timestamp;
otherwise it looks for it at the current namespace declared at the beginning of the class. other solution is to use it:
use Carbon\Carbon
$now = Carbon::now()->timestamp;
you can even assign it an alias:
use Carbon\Carbon as Time;
$now = Time::now()->timestamp;
hope it helps.