file.php

<?php
public function posts_by_day() {
  global  $json_api, $post;
    $days = array();
    add_filter( 'posts_where', array( $this, 'filter_where' ) );
    $query = new WP_Query( array(
    	'post_type' => array('post', 'imported_content' ),
    	'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,
	);
	
  }

public function filter_where( $where = '' ) {
	global $wpdb, $date_range;
	// posts for March 1 to March 15, 2010
	if( isset( $_GET['start_date'] ) && isset( $_GET['end_date'] ) )
		$where .= $wpdb->prepare( " AND post_date >= %s AND post_date < %s", $_GET['start_date'], $_GET['end_date'] );
	return $where;
}