Que erro e esse

Caros colegas

Ao converter uma variavel datetime para date assim

$var1 = date('d/n/Y' strtodate($var2)); o sistema não rodou mais e mostro a seguinte mensagem.

[Tue Jun 10 16:03:46 2014] [error] [client 10.10.20.174] PHP Warning: strtotime(): It is not safe to rely on the system's timezone settings. You are required to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/Sao_Paulo' for 'BRT/-3.0/no DST' instead in /var/www/html/pezinho_web/relatorios/rel_liberados.php on line 260, referer: http://desenv/pezinho_web/relatorios/pku_alterado.php

[Tue Jun 10 16:03:46 2014] [error] [client 10.10.20.174] PHP Warning: date(): It is not safe to rely on the system's timezone settings. You are required to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/Sao_Paulo' for 'BRT/-3.0/no DST' instead in /var/www/html/pezinho_web/relatorios/rel_liberados.php on line 260, referer: http://desenv/pezinho_web/relatorios/pku_alterado.php

O que esta a acontecer?
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