$a['date']+1
won't work as you can imagine.
You have to create a variable which contains the previous date, assuming your articles are ordered by date.
<?php
$_month = '0000-00';
foreach ($articles as $a) :
if (date('Y-m', strtotime($a['date'])) !== $_month) {
echo date('M',strtotime($a['date']));
}
...
$_month = date('Y-m', strtotime($a['date']));
endforeach;
?>