$repeatDay = "Tuesday";
 $followingDay = strtotime($startDateTime. " following $repeatDay");

$firstOccurrence=strtotime($start);
 $nextOccurence=strtotime("next $day", $firstOccurrence); //Set the first recurring date for this day
 while($nextOccurence<=strtotime($activeUntil)){
      echo date("M d, Y H:m:i",$nextOccurence)." | ";
      $nextOccurence=strtotime("next $day", $nextOccurence);
 }

// Assuming $startDateTime is a timestamp.
$followingDay = strtotime("next $repeatDay", $startDateTime);