[6] => Array
    (
        [id] => 4
        [count] => 2
        [dates] => Array
            (
                [0] => 1/27/2018
                [1] => 2/7/2018
            )

    )

[7] => Array
    (
        [id] => 5
        [dates] => Array
            (
                [0] => 1/20/2018
                [1] => 1/28/2018
                [2] => 2/10/2018
            )

    )

Solution - 1

Is it possible you can show me your site ?

Solution - 2

Hello, That's code is what you need exactly and we have add it to our demo site and working very good

/**
* Your requested function
*/

$value = array();
$output = array();

foreach( get_users() as $user ){
	$dates = array();
	$posts = get_posts( array('author' => $user->ID, 'posts_per_page' => -1) );
	foreach($posts as $p){
		$date = date('m/d/Y', strtotime($p->post_date));
		if( !in_array($date, $dates) ){
			array_push($dates, $date);
		}
	}

	$value['id'] = $user->ID;
	$value['count'] = count($dates);
	$value['dates'] = $dates;

	array_push($output, $value);
}

print_r( $output);

Array
(
    [0] => Array
        (
            [id] => 1
            [count] => 7
           [dates] => Array
                (
                    [0] => 02/17/2018
                    [1] => 1/16/2018
                    [2] => 12/5/2017
                    [3] => 11/15/2017
                    [4] => 10/29/2017
                    [5] => 10/19/2017
                    [6] => 09/19/2017
                )
        )
    [1] => Array
        (
            [id] => 2
            [count] => 5
            [dates] => Array
                (
                    [0] => 02/17/2018
                    [1] => 02/16/2018
                    [2] => 02/15/2018
                    [3] => 01/15/2018
                    [4] => 12/19/2017
                )
        )
)

Solution - 3

Here's one way, if I understand correctly, if you want to use the date query in WP_Query: We define this helper function:

/**
 * Helper function - Number of posts per user
 *
 * @uses WP_Query
 *
 * @param int $user_id User ID. Default 0.
 * @param array $dates Array of m/d/y dates. Default empty.
 * @return int $return Number of found posts. 
 */
function wpq_post_count_per_user( $user_id = 0, $dates = [] ) {
    $args = [
        'fields'                         => 'ids',
        'author'                        => (int) $user_id,
        'nopaging'                   => true,
        'ignore_sticky_posts'  => true,
        'date_query'                => [
            'relation' => 'OR',
        ],
    ];
    foreach( (array) $dates as $date ) {
        $args['date_query'][] = [
            'year'    => date( 'Y', strtotime( $date ) ),
            'month' => date( 'm', strtotime( $date ) ),
            'day'     => date( 'j', strtotime( $date ) ),
        ];
    }
    $query = new WP_Query( $args );
    return $query->found_posts;
}

$input = [
    [
        'id' => 1,
        'count' => 3,
        'dates' => [
            '1/20/2018',
            '1/22/2018',	
            '1/18/2018',
        ],
    ],
    [
        'id' => 2,
        'count' => 3,
        'dates' => [
            '1/28/2018',
            '1/29/2018',			
            '1/30/2018',
        ],
    ],
];

// Number of posts for a given user
echo wpq_post_count_per_user( $input[0]['id'], $input[0]['dates'] );

Solution - 4

$date_start = '2018-01-01';
$date_end = '2018-01-31';

$res = $wpdb->get_results( $wpdb->prepare("
	select count(ID) `count`, post_author, DATE(post_date) as day, DATE_FORMAT(post_date, '%c/%e/%Y') nice_day
	from {$wpdb->prefix}posts
	where
		post_date>=%s and post_date<=%s
		and post_type='post' and (post_status='publish' OR post_status='private')
	group by post_author, day
	order by day, post_author",
	$date_start, $date_end
));

$users = array();
foreach ( $res as $row )
{
	if ( !isset( $users[ $row->post_author ] ) )
		$users[ $row->post_author ] = array(
			'id' => $row->post_author,
			'count' => 0,
			'dates' => array(),
		);

		$users[ $row->post_author ]['count']++;
		$users[ $row->post_author ]['dates'][] = $row->nice_day;
}
$users = array_values( $users );