- date_create_from_format: documentation ( source)
- explode: documentation ( source)
<?php
$t = '1/99999';
list($m,$d)=explode("/",$t);$m%=12;$d%=365;$i=0;while($d>100)$d-=[31,28,31,30,31,30,31,31,30,31,30,31][$i++];$m+=$i;echo date_create_from_format("m/d","$m/$d")->format("n/j");