Caducar página

[yole]

Muchas gracias por tu ayuda.
Sucede que cuando lo ejecuto aparece lo siguiente:

Strict Standards: date() [function.date]: It is not safe to rely on the system's timezone settings. Please use the date.timezone setting, the TZ environment variable 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/Halifax' for '-4.0/no DST' instead in caducarpagina.php on line 39

que significa?