I dont know of any such function.. Actually this is the first time I ever hear of the Quantieme...
What you could do is write a simple switch statement...
get the month and the day....
switch($month){
case("jan");
$num=0;
case("feb");
$num=31;
case("mar");
$num=28+31;
...and so on...
$quantieme= $num + $dayofmonth
So for mar 9 2000
$quantieme = 28+31+9=68th day of 2000
Thats how I would do it...
You would probly want to stick in an extra If statement to check that there is only 28 days in Feb of that year...
Hope it helps...