Downloadable ics file (icalendar) snippet - Quick question

<?php
// iCalendar snippet
// by rolf ahlborn
// v0.0.1 - 2009-09-06
//
// ---------------------------------
//
// snippet will collect ressources/documents from defined folder(s)
// and creates an ics-file for downloading and subscribing your
// dates/events into iCal or Outlook or others
//
// ---------------------------------
//
// preparing:
// * copy this content entirely into a new snippet and name it: iCalendar
// * generate one or more folder with any ressources/documents
// * each document must have a pagetitle, a publication date and
//   normally an unpublishing date
// * create an document to place the snippet call into
//   i.e. [!iCalendar?startid=`16,18,52,57`!]
//   - if no startid is given, it will take the calling doc as startid
//   - give this document any title and set the Alias to the same name as
//     you define $icsFileName below, i.e. myCal.ics
//   - set the template to: (blank)
//   - set the content type to: text/plain
//   - you may set the behaviour to attachment, but snippet will deliver
//     the propper mime-type header for text/calendar and content-disposition
//   - you may hide this document from navigation and create a link anywhere 
//     to offer the download to that file 
// * according to the OS preferences, you or your users will have to subscribe
//   either the address (URI) created by your snippet doument or import it locally
//   after downloading the file - let them know... in first case, their subscribed
//   calendars will be updated if you change your related MODx documents 


// startid can be provided in snippet parameters and a comma separated list
// default startid: doc itself
$startid = (isset($startid)) ? $startid : $modx->documentIdentifier;

// specify field to sort by, default menuindex
$sortBy = (isset($sortBy)) ? $sortBy : 'menuindex';

// sortdirection of collected docs, default ascending
$sortDirection = (isset($sortDirection)) ? $sortDirection : 'ASC';

// define iCalender title - will be used in your calendar application
$icalName = 'Calendrier d\'Événements Parole De Grace';

// define ics file name - filename for download file
$icsFileName = 'GraceCal.ics';

// define Timezome
// more information: http://en.wikipedia.org/wiki/ICalendar#cite_note-8
$icalTZ = 'America/Toronto';

// set an alarm before event starts, here 60 minutes to go
$alarmBefore = 60;

// define default event length in minutes, if no unpubdate is given
// let's say default fixed event length are 75 minutes
$duration = 75;


/////////////////////////////////////////////////////////////////////////////


// figure out the parent(s)
$parents = array();
// if $startid is given, check for comma
if(strpos($startid, ',') !== false) {
  // explode list to array
  $parents = explode(',', $startid);
} else {
  // otherwise create single-item array
  $parents[] = $startid;  
}

// collect the children(s) of parent(s)
$children = array();
foreach($parents as $parent){
  $childArr = $modx->getAllChildren($parent ,$sortBy, $sortDirection ,'pub_date,unpub_date,id,pagetitle,parent,deleted');
  foreach($childArr as $child){
  	// we only take undeleted docs
    if(($child['deleted'] == 0)){
      $children[] = $child;
    }
  }
}

// sort array, may be commented if sorting is bad
sort($children); 

// create output string in ical style
$output = "BEGIN:VCALENDAR\n";
$output .= "VERSION:2.0\n";
$output .= "PRODID:-//hi-tag//modx 0.0.1//EN\n"; // you may alter this
$output .= "X-WR-CALNAME:" . $icalName . "\n";
$output .= "X-WR-TIMEZONE:" . $icalTZ . "\n";

// create the events from children's title, pubdate and unpubdate
for ($c=0; $c < count($children) ; $c++){
  $evtdate = $children[$c]['pub_date'];
  $evtdateend = $children[$c]['unpub_date']; 
  
  // alternative DTEND for fixed event length or if no unpub date is given
  if($evtdateend == 0){  	  	
  	// get pubdate to get a time basis for calculation
    $y = date("Y", $evtdate);
    $m = date("m", $evtdate);
    $d = date("d", $evtdate);
    $h = date("H", $evtdate);
    $i = date("i", $evtdate);
    // add defined duration to pubdate
    $evtdateend = mktime($h, $i+$duration, 0, $m, $d, $y);
  }
  
  // add output string for every event
  $output .= "BEGIN:VEVENT\n";
  $output .= "DTSTART:" . date("Ymd", $evtdate) . "T" . date("His", $evtdate) . "Z\n";
  $output .= "DTEND:" . date("Ymd", $evtdateend) . "T" . date("His", $evtdateend) . "Z\n";
  
  $output .= "SUMMARY:" . $children[$c]['pagetitle'] . "\n";

  $output .= "BEGIN:VALARM\n";
  $output .= "ACTION:DISPLAY\n";
  $output .= "TRIGGER:-PT" . $alarmBefore . "M\n";
  $output .= "END:VALARM\n";
  
  $output .= "END:VEVENT\n";
} 

$output .= "END:VCALENDAR\n";

// prepare output as ics download file
header('Content-type: text/calendar');
header('Content-Disposition: attachment; filename="' . $icsFileName . '"');
echo $output;

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//hi-tag//modx 0.0.1//EN
X-WR-CALNAME:Calendrier d'Événements Parole De Grace
X-WR-TIMEZONE:America/Toronto
BEGIN:VEVENT
DTSTART:19691231T190000Z
DTEND:19691231T193330Z
SUMMARY:Conférence-Retraite Internationale
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER:-PT60M
END:VALARM
END:VEVENT
END:VCALENDAR