Setting Default Timezone - How?

I'm working on a 1and1 server and I'm getting this message on one of my calendar pages:

Strict Standards: 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/New_York' for 'EST/-5.0/no DST' instead in /homepages/42/d514133230/htdocs/adm/calendar_insert_execute.php on line 44

Strict Standards: 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/New_York' for 'EST/-5.0/no DST' instead in /homepages/42/d514133230/htdocs/adm/calendar_insert_execute.php on line 44

Strict Standards: 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/New_York' for 'EST/-5.0/no DST' instead in /homepages/42/d514133230/htdocs/adm/calendar_insert_execute.php on line 54

Strict Standards: 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/New_York' for 'EST/-5.0/no DST' instead in /homepages/42/d514133230/htdocs/adm/calendar_insert_execute.php on line 54

I put this in my header and I got the same error. What do I need to do to satisfy the system's concerns?

<?php
date_default_timezone_set('America/Chicago');
?>