date_default_timezone_set('Asia/Novosibirsk'); разве не должна исправить всё и поставить зону?
$month_now_last_day_in_time = strtotime('00:00 last day of Jan');$month_now_first_day = new datetime('first day of 0 month midnight');
$month_now_last_day = new datetime('last day of 0 month midnight');
$month_now_first_day_in_time = strtotime($month_now_first_day->format('Y-m-d H:i:s'));
$month_now_last_day_in_time = strtotime($month_now_last_day->format('Y-m-d H:i:s'));
$month_now_text = strftime("%B", $month_now_first_day_in_time);$month_now_first_day_in_time = strtotime('00:00 1st Jan');
$month_now_last_day_in_time = strtotime('00:00 last day of Jan');
$month_now_text = strftime("%B", $month_now_first_day_in_time);