Subtracting Dates, With Respect to Leap Years and Daylight Saving

[BladeMetal]

I have a script I'm writing where the user inputs their birthday and the program returns their age in whole years. I had this script, but during leap years and daylight saving, it could be thrown out.

$birth_date = $_POST['birthday_year'].'-'.$_POST['birthday_month'].'-'.$_POST['birthday_day'];
//creates birthday as a string
$manager_age = ((mktime() - strtotime($birth_date))/31556926);
//finds the difference as a timestamp and divides by the number a seconds in a normal year
echo floor($manager_age);
//prints the output

If someone could rewrite this for me so it actually works with leap years and daylight saving in mind it would be greatly appreciated.

Cheers
-Adam