I'd also suggest changing the first line from
$datestart=date ("Y-m-d",mktime (0,0,0,date("m"), date("d"), date("Y")-1));
to
$datestart=date('Y-m-d',strtotime('last year'));
In fact, you could rewrite the code to the point where date is only used once.
$datestart=strtotime('last year');
$nod = 0;
$nom = 1;
$noy = 0;
for ($i=0; $i<12; $i++){
$datestart = strtotime("$nod days $nom months $noy years", $datestart);
$display_date = date("Y-m-d", $datestart);
echo "<a href='tips.php?month=$display_date'>$display_date</a>  ";
}
And of course, that means there's only one date format string to adjust.