timezone error

My sites many pages gives the same timezone error. I am showing you some of the messages below. Kindly help me guys.

Login page after entering username and password

Warning: mktime() [function.mktime]: 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 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/classes/BxDolTemplate.php on line 299

Warning: mktime() [function.mktime]: 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 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/classes/BxDolForm.php on line 597

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 'America/Chicago' for 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/utils.inc.php on line 69

Warning: mktime() [function.mktime]: 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 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/classes/BxDolForm.php on line 597

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 'America/Chicago' for 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/utils.inc.php on line 69

Warning: Cannot modify header information - headers already sent by (output started at /home/circleor/public_html/inc/classes/BxDolTemplate.php:299) in /home/circleor/public_html/inc/design.inc.php on line 101

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Joining page 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 'America/Chicago' for 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/classes/BxDolProfileFields.php on line 1709

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 'America/Chicago' for 'CDT/-5.0/DST' instead in /home/circleor/public_html/inc/classes/BxDolProfileFields.php on line 1710

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http://brianshim.com/webtricks/fix-php-date-timezone-warnings/

 Thanks dear, it worked. My problem is solved but there is one error left. Logout error. Whenever admin or user clicks logout, give following error and the process remains incomplete as it shows members ac when url is opened in new tab.

it gives following message 

Parse error: syntax error, unexpected '<' in /home/circleor/public_html/logout.php on line 26

See the logout file in attachment.

Upload a new logout.php from a fresh download.

Did you not think it was relevant to tell us that your logout.php file has been modified with this code added at the end?

<!-- Facebook Conversion Code for Join your society -->
<script type="text/javascript">
var fb_param = {};
fb_param.pixel_id = '6016347734588';
fb_param.value = '0.01';
fb_param.currency = 'USD';
(function(){
var fpw = document.createElement('script');
fpw.async = true;
fpw.src = '//connect.facebook.net/en_US/fp.js';
var ref = document.getElementsByTagName('script')[0];
ref.parentNode.insertBefore(fpw, ref);
})();
</script>
<noscript><img height="1" width="1" alt="" style="display:none" src="https://www.facebook.com/offsite_event.php?id=6016347734588&amp;value=0.01&amp;currency=USD" /></noscript>

Is there no one to help me out here ????????????????

Nathan Paton also disappeared after asking for fresh file.

Your added JavaScript is breaking it - it shouldn't be there, especially with an unclosed <?php tag. Upload an unmodified copy of logout.php to the site.

Nathan didn't mean to upload it here, use it to replace the one with the bad code on your site.