DST problem demo

readme.txt

For timezone 'Australia/Melbourne':
2018-03-31 10:00:00
+ 24*60*60        == 2018-04-01 09:00:00
strtotime(+1 day) == 2018-04-01 10:00:00
DateTime->add(1d) == 2018-04-01 10:00:00
2018-10-06 10:00:00
+ 24*60*60        == 2018-10-07 11:00:00
strtotime(+1 day) == 2018-10-07 10:00:00
DateTime->add(1d) == 2018-10-07 10:00:00

For timezone 'Europe/Moscow':
2018-03-31 10:00:00
+ 24*60*60        == 2018-04-01 10:00:00
strtotime(+1 day) == 2018-04-01 10:00:00
DateTime->add(1d) == 2018-04-01 10:00:00
2018-10-06 10:00:00
+ 24*60*60        == 2018-10-07 10:00:00
strtotime(+1 day) == 2018-10-07 10:00:00
DateTime->add(1d) == 2018-10-07 10:00:00

test.php

<?php
//
// According to https://www.timeanddate.com/time/change/australia
// DST events are on 
//  1 April 2018, 03:00:00 and
//  7 October 2018, 02:00:00
//
function test($timezone)
{
    date_default_timezone_set($timezone); // affects both ts and DateTime funcs

    echo "\nFor timezone '{$timezone}':\n";
    addAndShow(strtotime('2018-03-31 10:00:00'));
    addAndShow(strtotime('2018-10-06 10:00:00'));
}

function addAndShow($ts)
{
    echo date('Y-m-d H:i:s', $ts)."\n";
    echo '+ 24*60*60        == '.date('Y-m-d H:i:s', $ts + 86400)."\n";
    echo 'strtotime(+1 day) == '.date('Y-m-d H:i:s', strtotime('+1 day', $ts))."\n";
    $o = (new DateTime)->setTimestamp($ts)->add(new DateInterval('P1D'));
    echo 'DateTime->add(1d) == '.$o->format('Y-m-d H:i:s')."\n";
}

test('Australia/Melbourne');
test('Europe/Moscow');