Eloquent的问题

I wanna query the users who created at 2016-01-10. So code in Laravel 5.1 like this:

$day1 = new Carbon();
$day1->year = 2016;
$day1->month = 1;
$day1->day = 10;

$day2 = new Carbon();
$day2->year = 2016;
$day2->month = 1;
$day2->day = 11;
User::whereBetween('created_at', [$day1->startOfDay(),     $day2->startOfDay()])->get();

The codes above works fine. But this normally way, does not:

User::whereBetween('created_at', [$day1->startOfDay(), $day1->endOfDay()])->get();

So what is the problem? thanks a lot.