[Plugin: GD Star Rating] timezone settings WARNING!?!

I get this error on my website:

PHP Strict Standards: strtotime() [function.strtotime]: 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/Denver’ for ‘MDT/-6.0/DST’ instead in /home/xxx/public_html/xxx.com/wp-content/plugins/gd-star-rating/css/gdsr.css.php on line 52

Thanks for help and supports

http://wordpress.org/extend/plugins/gd-star-rating/