<?php
//function widget($args, $instance)
$category_id = intval($instance['category_id']);
$myposts = get_posts("numberposts=-1&offset=0&category=" . $category_id . "&orderby=date&order=DESC");
$display_format = "F Y";
$compare_format = "Ym";
$previous_year_month_display = "";
$previous_year_month_value = "";
$count = 0;
$interval = "month";
$previous_year = "";
$previous_month = "";
$category_info = get_term_by("id", $category_id, "category");
$category = $category_info->slug;

foreach($myposts as $post) {
    $post_date = strtotime($post->post_date);
    $current_year_month_display = date_i18n($display_format, $post_date);
    $current_year_month_value = date($compare_format, $post_date);
    $current_year = date("Y", $post_date);
    $current_month = date("m", $post_date);

    if ($previous_year_month_value != $current_year_month_value) {
        if ($count > 0) {
            $url = $this->create_url($interval, $previous_year, $previous_month, $category_id, $category);
            echo "<li><a href=\"". $url . "\">" . $previous_year_month_display . "</a>";
                echo  " (" . $count . ")";
            echo "</li>";
        }
        $count = 0;
    }
    $count++;
    $previous_year_month_display = $current_year_month_display;
    $previous_year_month_value = $current_year_month_value;
    $previous_year = $current_year;
    $previous_month = $current_month;

}