ya encontre la solucion el codigo quedaria asi:
Código PHP:
$at = strtotime ("december 21 2009 ");
$at1 = strtotime ("january 31 2010 ");
$atok= date("m/d/Y", $at);
$atok1= date("m/d/y", $at1);
$at2 = strtotime ("january 01 2010 ");
$at3 = strtotime ("january 31 2010 ");
$atok10= date("m/d/Y", $at2);
$atok11= date("m/d/y", $at3);
gracias