For example to get the following

Every 20th Monday of the year, forever:
DTSTART;TZID=US-Eastern:19970519T090000
RRULE:FREQ=YEARLY;BYDAY=20MO

==> (1997 9:00 AM EDT)May 19
(1998 9:00 AM EDT)May 18
(1999 9:00 AM EDT)May 17

One could get the first Monday of the year, the it is fairly easy to get the 20th !

<?php

global $amr_day_of_week_no;
$amr_day_of_week_no = array (
 'MO' => 1,
 'TU' => 2,
 'WE' => 3,
 'TH' => 4,
 'FR' => 5,
 'SA' => 6,
 'SU' => 0
 );    

 function amr_goto_byday ($dateobj, $byday, $sign)    {
 global $amr_day_of_week_no;    
 $dayofweek = $dateobj->format('w'); /* 0=sunday, 6 = saturday */
 if ($dayofweek == '-1') $dayofweek = get_oldweekdays($dateobj); /* php seems to break around 1760   */
$target     = $amr_day_of_week_no[$byday]; /*  mo=1 ,su=7  */
 $adjustment = $target - $dayofweek;
 if ($sign === '+') {
 if ($adjustment < 0) $adjustment = $adjustment + 7;        
 }
 else if ($adjustment > 0) $adjustment = $adjustment-7;    
 $d2 = new DateTime();
 $d2 = clone ($dateobj);         
 date_modify ($d2,$adjustment.' days');    
 return ($d2);
 }    

 /* --------Test data ------------------------- */
$d[] = new DateTime('2009-12-25');
$d[] = new DateTime('2009-12-28');
$d[] = new DateTime('2009-12-29');
$d[] = new DateTime('2009-12-30');
$d[] = new DateTime('2009-12-31');
$d[] = new DateTime('2010-01-01');
$d[] = new DateTime('2010-01-02');
$d[] = new DateTime('2010-01-03');
$d[] = new DateTime('2010-01-04');
$d[] = new DateTime('2010-01-05');
$d[] = new DateTime('2010-01-06');
$d[] = new DateTime('2010-01-07');
$d[] = new DateTime('2010-01-08');
$d[] = new DateTime('2010-01-09');
$d[] = new DateTime('2010-01-10');
$d[] = new DateTime('2010-01-11');

echo '<table>';    
foreach (array ('TU','WE','TH','FR','SA','SU','MO') as $day) {
 echo '<tr><td> Aiming for '.$day.'</td><td>If not this, then next</td><td>If not this, then prev</td></tr>';                
 foreach ($d as $i => $d2)    {
 $d3 = amr_goto_byday ($d2, $day, '+');
 $d4 = amr_goto_byday ($d2, $day, '-');
 $s = $d2->format('Y m d l');
 echo '<tr><td> '.$s.'</td><td>'.$d3->format('l,Y m d').'</td><td>'.$d4->format('l,Y m d').'</td></tr>';
 }    
}
echo '</table>';   
  
?>

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I now use the following code, which appears to work when comparing the weekdays using http://www.searchforancestors.com/utility/dayofweek.html

$w = $dateobj->format('w');
if ($w == '-1') $w = get_oldweekdays($dateobj);

function get_oldweekdays ($d) {
   $dummy = new DateTime();
   $dummy = clone ($d);
   date_modify($dummy,'+91500 weeks');
   /* add weeks to get us back to a "working" date -
   a guess from when the date started breaking, plus some extra to be safe */
   $w = $dummy->format('w');
 return($w);
}

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 /* Year if format YYYY, Day in year 1 to 366 */
function dayofyear2date( $year, $DayInYear ) {
$d = new DateTime($year.'-01-01');
date_modify($d, '+'.($DayInYear-1).' days');
return ($d);
}

I’m tired of seeing Unix time stamp code examples that break with older dates. Since my ical calendar scripts can potentially hold recurring dates that are anniversaries and birthdays, my scripts need to cope with dates earlier than 1970, and with timezones and daylight saving, so I use the DateTime class exclusively now.

Test Script To Check Function

<?php

function dayofyear2date( $year, $DayInYear ) {
 $d = new DateTime($year.'-01-01');
 date_modify($d, '+'.($DayInYear-1).' days');
return ($d);
}

$d[] = new DateTime('1950-12-31');
$d[] = new DateTime('1970-01-01');
$d[] = new DateTime('2010-01-01');
$d[] = new DateTime('2010-12-27');
$d[] = new DateTime('2010-12-31');

foreach ($d as $i => $d2)    {
 $dayofyear = $d2->format('z')+1;
 $date = dayofyear2date (
 $d2->format('Y'),
 $dayofyear ); /* note z returns days from 0 */
 echo '<br /><b>'.$d2->format('Y m d')
  .'</b>&nbsp;&nbsp;'.$dayofyear.'&nbsp;&nbsp;<br />'
  .$date->format('Y m d');
}
?>

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