I think I got it, or at least it is working now. I changed into the datepicker field options from 14/09/2915 to d/m/Y (I wrote it manually, I don’t know why selecting the 14/09/2015 option didn’t work).
And now using this code is working fine:
<div class="col-sm-3">
<?php
if(get_field('fecha_del_evento'))
{
$datetime = DateTime::createFromFormat('d/m/Y', get_field('fecha_del_evento'));
$fecha_dia = $datetime->format('d');
$fecha_mes = $datetime->format('M');
}
?>
<div class="eventos row">
<div class="col-xs-3">
<div class="fecha">
<p class="mes"><?php echo $fecha_mes; ?></p>
<p class="dia"><?php echo $fecha_dia; ?></p>
</div>
</div>
<div class="col-xs-9">
<?php the_content (); ?>
</div>
</div>
</div>
