diff -Nuriw a/calendar/lib.php b/calendar/lib.php
--- a/calendar/lib.php	2014-10-24 14:13:46.000000000 +0100
+++ b/calendar/lib.php	2014-10-24 14:14:33.000000000 +0100
@@ -249,7 +249,7 @@
 
     // These are used for DB queries, so we want unixtime, so we need to use Gregorian dates.
     $display->tstart = make_timestamp($gy, $gm, $gd, $gh, $gmin, 0);
-    $display->tend = $display->tstart + ($display->maxdays * DAYSECS) - 1;
+    $display->tend = $display->tstart + strtotime("+$display->maxdays day",0) - 1; 
 	
     // Align the starting weekday to fall in our display range
     // This is simple, not foolproof.
@@ -318,9 +318,10 @@
     }
 
     // Now display all the calendar
-    $daytime = $display->tstart - DAYSECS;
+
+    $daytime = strtotime( '-1 day',$display->tstart); 
     for($day = 1; $day <= $display->maxdays; ++$day, ++$dayweek) {
-        $daytime += DAYSECS;
+	$daytime = strtotime('+1 day', $daytime);
 	 if($dayweek > $display->maxwday) {
             // We need to change week (table row)
             $content .= '</tr><tr>';
@@ -534,7 +535,7 @@
 
     // This works correctly with respect to the user's DST, but it is accurate
     // only because $fromtime is always the exact midnight of some day!
-    $display->tend = usergetmidnight($display->tstart + DAYSECS * $display->range + 3 * HOURSECS) - 1;
+	$display->tend = usergetmidnight( strtotime('+1 day',$display->tstart) * $display->range + 3 * HOURSECS) - 1;
 
     // Get the events matching our criteria
     $events = calendar_get_events($display->tstart, $display->tend, $users, $groups, $courses);
@@ -957,8 +958,8 @@
         case 'day':
             $days = calendar_get_days();
 
-            $prevtimestamp = $time - DAYSECS;
-            $nexttimestamp = $time + DAYSECS;
+            $prevtimestamp = strtotime('-1 day',$time);
+            $nexttimestamp = strtotime('+1 day',$time);
 		
             $prevdate = $calendartype->timestamp_to_date_array($prevtimestamp);
             $nextdate = $calendartype->timestamp_to_date_array($nexttimestamp);
@@ -1675,7 +1676,7 @@
         // Check if we will still be on the same day.
         if ($usermidnightstart == $usermidnightend) {
             // Check if we are running all day.
-            if ($event->timeduration == DAYSECS) {
+            if ($event->timeduration == strtotime('1 day',0)) { 
                 $time = get_string('allday', 'calendar');
             } else { // Specify the time we will be running this from.
                 $datestart = calendar_time_representation($starttime);
@@ -1704,7 +1705,7 @@
             $timeend = calendar_time_representation($event->timestart + $event->timeduration);
 
             // Set printable representation.
-            if ($now >= $usermidnightstart && $now < ($usermidnightstart + DAYSECS)) {
+            if ($now >= $usermidnightstart && $now < (strtotime('+1 day', $usermidnightstart))) {
                 $url = calendar_get_link_href(new moodle_url(CALENDAR_URL . 'view.php', $linkparams), 0, 0, 0, $endtime);
                 $eventtime = $timestart . ' <strong>&raquo;</strong> ' . html_writer::link($url, $dayend) . $timeend;
             } else {
@@ -2751,7 +2752,7 @@
      * @return int tomorrow timestamp
      */
     public function timestamp_tomorrow() {
-        return $this->time + DAYSECS;
+    	return strtotime('+1 day',$this->time); 
  	}
     /**
      * Adds the pretend blocks for the calendar

diff -Nuriw a/calendar/renderer.php b/calendar/renderer.php
--- a/calendar/renderer.php	2014-10-24 13:16:41.000000000 +0100
+++ b/calendar/renderer.php	2014-10-24 13:20:23.000000000 +0100
@@ -89,10 +89,9 @@
         if ($CFG->calendar_customexport) {
             $a = new stdClass();
             $now = time();
-            $time = $now - $CFG->calendar_exportlookback * DAYSECS;
-		
+            $time = $now - strtotime("+ $CFG->calendar_exportlookback day",0);
             $a->timestart = userdate($time, get_string('strftimedatefullshort', 'langconfig'));
-            $time = $now + $CFG->calendar_exportlookahead * DAYSECS;
+            $time = $now + strtotime("+$CFG->calendar_exportlookahead day",0);
             $a->timeend = userdate($time, get_string('strftimedatefullshort', 'langconfig'));
             $output .= html_writer::empty_tag('input', array('type' => 'radio', 'name' => 'preset_time', 'id' => 'pt_custom', 'value' => 'custom'));
             $output .= html_writer::tag('label', get_string('customexport', 'calendar', $a), array('for' => 'pt_custom'));
@@ -439,7 +438,7 @@
 
         // These are used for DB queries, so we want unixtime, so we need to use Gregorian dates.
         $display->tstart = make_timestamp($gy, $gm, $gd, $gh, $gmin, 0);
-        $display->tend = $display->tstart + ($display->maxdays * DAYSECS) - 1;
+        $display->tend = $display->tstart + strtotime("+$display->maxdays day",0) - 1;
 	 
         // Align the starting weekday to fall in our display range
         // This is simple, not foolproof.
@@ -504,9 +503,9 @@
             $weekend = intval($CFG->calendar_weekend);
         }
 
-        $daytime = $display->tstart - DAYSECS;
+        $daytime = strtotime( '-1 day',$display->tstart);
         for ($day = 1; $day <= $display->maxdays; ++$day, ++$dayweek) {
-            $daytime = $daytime + DAYSECS;
+            $daytime = strtotime('+1 day', $daytime);
             if($dayweek > $display->maxwday) {
                 // We need to change week (table row)
                 $table->data[] = $row;