diff --git a/calendar/sitemgr/class.module_calendar_list.inc.php b/calendar/sitemgr/class.module_calendar_list.inc.php
index 42823ceb3a..9149736f8c 100644
--- a/calendar/sitemgr/class.module_calendar_list.inc.php
+++ b/calendar/sitemgr/class.module_calendar_list.inc.php
@@ -254,20 +254,19 @@ class module_calendar_list extends Module
 
 		if (($arguments['acceptDateParam']) && (get_var('date',array('POST','GET'))))
 		{
-			$first = $start = (int) (strtotime(get_var('date',array('POST','GET'))) +
+			$first = (int) (strtotime(get_var('date',array('POST','GET'))) +
 					(60 * 60 * 24 * 7 * $dateOffset));
 		}
 		else
 		{
-			$first = $start = (int) ($this->bo->now_su +
+			$first = (int) ($this->bo->now_su +
 					(60 * 60 * 24 * 7 * $dateOffset));
 		}
 		if ($arguments['useWeekStart'])
 		{
-			$first = $this->ui->datetime->get_weekday_start(
-				adodb_date('Y',$start),
-				adodb_date('m',$start),
-				adodb_date('d',$start));
+			$start = Api\DateTime($first);
+			$start->setWeekStart();
+			$first = $start->format('ts');
 		}
 
 		$last = (int) ($first +
diff --git a/calendar/sitemgr/class.module_calendar_month.inc.php b/calendar/sitemgr/class.module_calendar_month.inc.php
index 0df1ca33b7..8e0ad35dac 100644
--- a/calendar/sitemgr/class.module_calendar_month.inc.php
+++ b/calendar/sitemgr/class.module_calendar_month.inc.php
@@ -239,10 +239,9 @@ class module_calendar_month extends Module
 			$start = (int) ($this->bo->now_su +
 					(60 * 60 * 24 * 7 * $dateOffset));
 		}
-		$first = $this->ui->datetime->get_weekday_start(
-					adodb_date('Y',$start),
-					adodb_date('m',$start),
-					adodb_date('d',$start));
+		$start = new Api\DateTime($start);
+		$start->setWeekstart();
+		$first = $start->format('ts');
 		$last = strtotime("+$weeks weeks",$first) - 1;
 
 		if ($arguments['showTitle'])
diff --git a/calendar/sitemgr/class.module_calendar_planner.inc.php b/calendar/sitemgr/class.module_calendar_planner.inc.php
index c448f0606a..165f0a2ecf 100644
--- a/calendar/sitemgr/class.module_calendar_planner.inc.php
+++ b/calendar/sitemgr/class.module_calendar_planner.inc.php
@@ -198,7 +198,7 @@ class module_calendar_planner extends Module
 		}
 		$this->arguments['resources']['options'] = array_unique($this->arguments['resources']['options']);
 		$this->arguments['resources']['multiple'] = count($this->arguments['resources']['options']) ? 4 : 0;
-		
+
 
 		return parent::get_user_interface();
 	}
@@ -281,7 +281,9 @@ class module_calendar_planner extends Module
 			}
 			elseif ($ui->planner_view == 'week' || $ui->planner_view == 'weekN')	// weeekview
 			{
-				$ui->first = $ui->datetime->get_weekday_start($ui->year,$ui->month,$ui->day);
+				$start = new Api\DateTime($ui->date);
+				$start->setWeekstart();
+				$ui->first = $start->format('ts');
 				$ui->last = $ui->bo->date2array($this->first);
 				$ui->last['day'] += ($ui->planner_view == 'week' ? 7 : 7 * $ui->cal_prefs['multiple_weeks'])-1;
 				$ui->last['hour'] = 23; $ui->last['minute'] = $ui->last['sec'] = 59;