// En el tpl.php
<? 
  $fecha_1=explode ('T', ($node->field_dia_y_hora[0]['value']));
  $fecha=explode ('-', $fecha_1[0]);
  $fecha = mktime(0, 0, 0, $fecha[1], $fecha[2], $fecha[0] );
?>
  <span class="post-month"><?php print (date('M', $fecha)) ?></span>
  <span class="post-day"><?php print (date('d', $fecha)) ?></span>
/* sytle.css */
.post-date-agenda {
  float:left;
  width:55px;
  height:40px;
  margin-right:10px;
  border-right:1px dotted #ccc;
  background-image: url(images/calendario1.png);
  background-repeat:no-repeat;
  background-position:left top;
}
 
.post-date-agenda .post-month {
  display:block;
  margin-left:0px;
  padding-top: 5px;
  font-size: 8px;
  line-height: 10px;
  text-transform: uppercase;
  text-align: center;
  color: #fff;
}
.post-date-agenda .post-day {
  display:block;
  margin-left:0px;
  padding-top: 2px;
  font-size: 14px;
  font-weight:bold;
  text-transform: uppercase;
  text-align: center;
  line-height: 18px;
  color: #333;
}