nextcloud/apps/calendar/templates/part.getcal.php

<?php
$calendars = OC_Calendar_Calendar::allCalendars(OC_User::getUser(), 1);
$events = OC_Calendar_Calendar::allCalendarObjects($calendars[0]['id']);
$select_year = $_GET["year"];
$return_events = array();
$user_timezone = OC_Preferences::getValue(OC_USER::getUser(), "calendar", "timezone", "Europe/London");
foreach($events as $event)
{
	if ($select_year != substr($event['startdate'], 0, 4))
		continue;
	$start_dt = new DateTime($event['startdate'], new DateTimeZone('UTC'));
	$start_dt->setTimezone(new DateTimeZone($user_timezone));
	$end_dt = new DateTime($event['enddate'], new DateTimeZone('UTC'));
	$end_dt->setTimezone(new DateTimeZone($user_timezone));
	$year  = $start_dt->format('Y');
	$month = $start_dt->format('n') - 1; // return is 0 based
	$day   = $start_dt->format('j');
	$hour  = $start_dt->format('G');

	// hack
	if (strstr($event['calendardata'], 'DTSTART;VALUE=DATE:')) {
		$hour = 'allday';
	}
	$return_event = array();
	foreach(array('id', 'calendarid', 'objecttype', 'repeating') as $prop)
	{
		$return_event[$prop] = $event[$prop];
	}
	$return_event['startdate'] = $start_dt->format('Y-m-d H:i');
	$return_event['enddate'] = $end_dt->format('Y-m-d H:i');
	$return_event['description'] = $event['summary'];
	if (isset($return_events[$year][$month][$day][$hour]))
	{
		$return_events[$year][$month][$day][$hour][] = $return_event;
	}
	else
	{
		$return_events[$year][$month][$day][$hour] = array(1 => $return_event);
	}
}
echo json_encode($return_events);
?>