file.php

<?php
public function posts_by_day() {
    global  $json_api, $post;
    $days = array();
    $query = new WP_Query( array(
    	'post_type' => array( ),
    	'posts_per_page' => -1,
    ) );
    if( $query->have_posts() ): while( $query->have_posts() ): $query->the_post();
    	if( !isset( $days[ get_the_time( 'Y-m-d' ) ] ) )
    		$days[ get_the_time( 'Y-m-d' ) ] = array();
    	$days[ get_the_time( 'Y-m-d' ) ][] = $post;
    endwhile;
    endif;

    $days = array_values( $days );
    return array(
		"posts" =>  $days,
	);
	
  }