diff --git a/calendar/lib.php b/calendar/lib.php
index 6892724e8a3..b4358420709 100644
--- a/calendar/lib.php
+++ b/calendar/lib.php
@@ -932,8 +932,8 @@ function calendar_top_controls($type, $data) {
         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);
@@ -1679,7 +1679,7 @@ function calendar_format_event_time($event, $now, $linkparams = null, $usecommon
             $timeend = calendar_time_representation($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 {
@@ -2775,7 +2775,7 @@ class calendar_information {
      * @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 --git a/calendar/renderer.php b/calendar/renderer.php
index 4a0e6cbd0fb..688aab0604e 100644
--- a/calendar/renderer.php
+++ b/calendar/renderer.php
@@ -503,9 +503,9 @@ class core_calendar_renderer extends plugin_renderer_base {
             $weekend = intval($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;