change date to function api_get_utc_datetime for dashboard teacher report chart refs #5713

plugin/dashboard/block_teacher_graph/block_teacher_graph.class.php

@@ -117,8 +117,9 @@ class BlockTeacherGraph extends Block {
 				$username = $teacher_info['username'];
 				$time_by_days = array();
 				foreach ($a_last_week as $day) {
-					$start_date = date('Y-m-d 00:00:00', $day);
-                    $end_date   = date('Y-m-d 23:59:59', $day);
+					// day is received as y-m-d 12:00:00
+					$start_date = api_get_utc_datetime($day);
+					$end_date = api_get_utc_datetime($day+(3600*24-1));
 
 					$time_on_platform_by_day = Tracking::get_time_spent_on_the_platform($user_id, 'custom', $start_date, $end_date);
 					$hours = floor($time_on_platform_by_day / 3600);