Have some troubles: mktime(): Argument #5 ($day) must be of type ?int, string given

LINE:161
FILE:/var/www/u0043737/public_html/wakepeople.ru/lib/lib.php
TRACE:
/var/www/u0043737/public_html/wakepeople.ru/lib/lib.php161mktime
/var/www/u0043737/public_html/wakepeople.ru/class/book/class.bookobject.php88mySQLdate
/var/www/u0043737/public_html/wakepeople.ru/megabook_lib.php20calendar
/var/www/u0043737/public_html/wakepeople.ru/megabook.php33showObjectCalendar