Panowie, spójrzcie proszę w poniższy kalendarz:
w linii:

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$days_in_this_month = cal_days_in_month(CAL_GREGORIAN, $month, $year);
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oraz w linii

[PHP] pobierz, plaintext 
$days_in_last_month = cal_days_in_month(CAL_GREGORIAN, $month_last, $year_last);
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rzuca błędem Warning: cal_days_in_month() [function.cal-days-in-month]: invalid date. in

Problem pojawia się , gdy próbuje w kalendarzu przewinąć o kolejne 3 miesiące.

[PHP] pobierz, plaintext 
function draw_cal($id_item, $month, $year, $manage_type="Wolne"){
	global $lang;
 
	$month=sprintf("%02s", $month);
	$today_timestamp	=   mktime(0,0,0,date('m'),date('d'),date('Y'));
	$this_month 		= 	getDate(mktime(0, 0, 0, $month, 1, $year));
	$first_week_day 	= $this_month["wday"];
	$days_in_this_month = cal_days_in_month(CAL_GREGORIAN, $month, $year);
	$day_counter_tot	=	0;
 
	$month_last	= $month - 1;
	$year_last	= $year;
	if($month_last < 1){
		$month_last = 12;
		$year_last = $year - 1;	
	}
	$days_in_last_month = cal_days_in_month(CAL_GREGORIAN, $month_last, $year_last);
 
	if($show_week_num)	$list_day_titles='<li class="weeknum_spacer"></li>';
 
	if(AC_START_DAY=="sun"){
		for($k=0; $k<7; $k++){
			$weekday = mb_substr($lang["day_".$k.""],0,1,'UTF-8');
			$list_day_titles.='<li class="cal_weekday"> '.$weekday.'</li>';
		}
	}else{
		if ($first_week_day == 0)	$first_week_day =7;
		for($k=1; $k<=7; $k++){
			if($k==7) 	$weekday = mb_substr($lang["day_0"][0],0,1,'UTF-8');
			else		$weekday = mb_substr($lang["day_".$k.""],0,1,'UTF-8');
			$list_day_titles.='<li title="'.$lang["day_".$k.""].'"> '.$weekday.'</li>';
		}
	}
 
	$j=1;
	if(AC_START_DAY=="sun")	
		$first_week_day_start = $first_week_day;	# start niedziela
 
	else
		$first_week_day = $first_week_day-1;	# start poniedziałek
 
	$row_counter=0;
 
	if($first_week_day!=7){
		if($show_week_num)	$list_days.='<li class="weeknum">-</li>';
		$last_month_start_num=$days_in_last_month-$first_week_day+1;
		for($week_day = 0; $week_day < $first_week_day; $week_day++){
			$list_days.='<li class="cal_empty">'.$last_month_start_num.'</li>';   
			++$last_month_start_num;
			++$j;
			++$day_counter_tot;
 
			if($day_counter_tot % 7==1) ++$row_counter;
		}
	}
	$week_day=$j;
 
	$booked_days=array();
	$pdo = new db_connect();
	$sql = $pdo->prepare("
	SELECT 
		t1.the_date,
		t2.class,
		t2.desc_pl AS the_state
	FROM 
		bookings AS t1
		LEFT JOIN bookings_states AS t2 ON t2.id=t1.id_state
	WHERE 
		t1.id_item=".$id_item." 
		AND MONTH(t1.the_date)=".$month." 
		AND YEAR(t1.the_date)=".$year."
	");
	$sql->execute();
 
	while($row = $sql->fetch()){
		$booked_days[$row["the_date"]]=array("class"=>$row["class"],"state"=>$row["the_state"]);
	}
 
 
	for($day_counter = 1; $day_counter <= $days_in_this_month; $day_counter++){
 
		$day_classes 	=	"";
		$day_title_state=	" - ".$lang["available"];
 
		$day_classes.=' clickable';
		$date_timestamp =   mktime(0,0,0, $month,($day_counter),$year);
 
		$week_num=date("W",$date_timestamp);
		if($week_num!=$last_week_num){
		}
 
		if($date_timestamp==$today_timestamp)	$day_classes.=' today';
 
		$date_db		=	$year."-".sprintf("%02s",$month)."-".sprintf("%02s",$day_counter);
 
        if(AC_DATE_DISPLAY_FORMAT=="pl")	$date_format	=	$month."/".$day_counter."/".$year;
        else 			        			$date_format	=	$day_counter."/".$month."/".$year;
 
		if(array_key_exists($date_db,$booked_days)){
			$day_classes.=" ".$booked_days[$date_db]["class"];
			$day_title_state=" - ".$booked_days[$date_db]["state"];
		}		
 
		if( $date_timestamp<$today_timestamp){
			$day_classes.=" past";
 
			if(AC_ACTIVE_PAST_DATES=="off"){
				$day_classes=str_replace(' clickable','',$day_classes);
			}
		}
 
		$getdate=getdate($date_timestamp);
		$day_num=$getdate["wday"]+1;
		if ($day_num % 7 == 1)		$day_classes.=' weekend';
		elseif ($day_num % 6 == 1)	$day_classes.=' weekend';
 
		$list_days .= '
		<li class="'.$day_classes.' "  id="date_'.$date_db.'" title="'.$date_format.$day_title_state.'" data-date="'.$date_format.'">'.$day_counter.'</li>';
 
		$week_day %= 7;
		++$week_day;
		++$day_counter_tot;
 
		if($show_week_num){
			if ($week_day==1) $list_days .= '<li class="weeknum">'.$week_num.'</li>';
		}
		$last_week_num=$week_num;
		if($day_counter_tot % 7==1) ++$row_counter;
	}
	$next_month_day=1;
 
	while($row_counter<6){
 
		for($till_day = $week_day; $till_day <=7; $till_day++){
			$list_days .= '<li class="cal_empty">'.$next_month_day.'</li>'; 
			++$next_month_day;  
			++$day_counter_tot;
 
		if($day_counter_tot % 7==1) ++$row_counter;
		}
		$week_day=1;
 
	}
 
	if($week_day > 1){
		for($till_day = $week_day; $till_day <=7; $till_day++){
			$list_days .= '<li class="cal_empty">'.$next_month_day.'</li>'; 
			++$next_month_day;  
			++$day_counter_tot;
		}
	}
 
	$the_cal='
	<div id="'.$month.'_'.$year.'" class="cal_title">'.$lang["month_".$month.""].' '.$year.'</div>
	<ul class="cal_weekday">
		'.$list_day_titles.'
	</ul>
	<ul>
		'.$list_days.'
	</ul>
	<div class="clear"></div>
	';
	return $the_cal;
}
 
//	plik językowy
include('lang/pl.lang.php');
 
 
$id_item	= $_GET['id_item'];
$the_month	= $_GET['month'];
$the_year	= $_GET['year'];
 
 
if($id_item=="") 	die("no item defined");
if($the_month=="") 	die("no month defined");
if($the_year=="") 	die("no year defined");
 
// Wyświetlenie kalendarza.
echo draw_cal($id_item,$the_month,$the_year);
[PHP] pobierz, plaintext 

No to sprawdź sobie co pakujesz do tych funkcji, jakie wartości.

jak to co pakuje do tej funkcji:

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 $month = 06
 
 $year = 2017 
[PHP] pobierz, plaintext 

Taa??
to zrób

[PHP] pobierz, plaintext 
var_dump(CAL_GREGORIAN, $month, $year);
$days_in_this_month = cal_days_in_month(CAL_GREGORIAN, $month, $year);
[PHP] pobierz, plaintext 

Po załadowaniu kalendarza:
int(0) string(2) "06" string(4) "2017"
int(0) string(2) "07" string(4) "2017"
int(0) string(2) "08" string(4) "2017"

po kliknięciu w kolejne miesiące:
int(0) string(2) "51" string(4) "2018" // i tu się coś pierdzieli
int(0) string(2) "71" string(4) "2018"
int(0) string(2) "81" string(4) "2018"

No widzisz... naucz się debugować swój własny kod...