$startTime = strtotime( '2010-05-01 12:00' );
$endTime = strtotime( '2010-05-10 12:00' );

// Loop between timestamps, 24 hours at a time
for ( $i = $startTime; $i <= $endTime; $i = $i + 86400 ) {
  $thisDate = date( 'Y-m-d', $i ); // 2010-05-01, 2010-05-02, etc
}