wp-get-dates.php

<?php

/**
 *  Returns an array of dates in which content has been published
 *
 *  @param string   $date_type	'day', 'month', or 'year'
 *  @param string   $format     PHP date format string
 *  @param string   $post_type  Post type to get dates for
 *  @return array 		Array of dates in which content has been published
 */
function get_dates($date_type = 'year', $post_type = 'post', $format = false ) {

	$display_format = "Y";
	$compare_format = "Y";

	if ($date_type == 'month') {
		$display_format = "M Y";
    	$compare_format = "Y-m";
	} else if ($date_type == 'day') {
		$display_format = "M j, Y";
    	$compare_format = "Y-m-d";
	}

    if ($format) $display_format = $format;

	
    $posts = get_posts(array('numberposts' => -1, 'post_type' => $post_type));
    $previous_display = "";
    $previous_value = "";        
    $count = 0;

    $dates = array();

    foreach($posts as $post) {
        $post_date = strtotime($post->post_date);
        $current_display = date_i18n($display_format, $post_date);
        $current_value = date($compare_format, $post_date);

        if ($previous_value != $current_value) {
        	$dates[$current_value] = $current_display;
        }
        $previous_display = $current_display;
        $previous_value = $current_value;

    }
    return $dates;
}