I‘m getting the following errors in my log file whenever I use phpinfo() function:

[10-Jan-2011 18:39:08] PHP Warning: phpinfo(): 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/Chicago’ for ‘CST/-6.0/no DST’ instead in /home/servers/http/dev/testpc.php on line 2

How do I fix this problem under UNIX / Linux php running Apache or lighttpd web server?