<?php $cur_y = get_the_time('Y'); ?>
<?php $cur_m = get_the_time('m'); ?>
<?php $off = 1; ?>
<?php if($cur_m == 12) {
  $next_m = 1;
  $next_y = $cur_y + $offset;
} elseif ($cur_m != 12) {
  $next_m = $cur_m + $off;
  $next_y = $cur_y;
} ?>

<?php if($cur_m == 1) {
  $last_m = 12;
  $last_y = $cur_y - $offset;
} elseif ($cur_m != 1) {
  $last_m = $cur_m - $off;
  $last_y = $cur_y;
} ?>

<?php $prev_m = $cur_m - $off; ?>
<?php $m_prev=query_posts('year=$prev_y&monthnum=$prev_m'); ?>
<?php wp_reset_query(); ?>
<?php $m_next=query_posts('year=$next_y&monthnum=$next_m'); ?>
<?php wp_reset_query(); ?>