Warning: strtotime() [function.strtotime]: It is not safe to rely on the system's timezone settings. You are *required* to...

I'm using XAMPP.
If you have problem with this error:
Warning: date() [function.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 'Asia/Krasnoyarsk' for '7.0/no DST' instead in C:\xampp\htdocs\guestbook\index.php on line 12.

you just put at the top of the index.php page.

After upgrading webservers php 5.2.9 to new stable 5.3.0 wordpress started to give following errors to every page, this error message appears:
Warning: strtotime() [function.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 'Europe/Helsinki' for 'EEST/3.0/DST' instead in /path/to/my/www/wp-includes/functions.php on line 35

In /wp/wp-includes/functions.php
Add the following before the first function:
date_default_timezone_set('UTC');