This message keeps on appearing on the BUY page of my website.

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 'America/Los_Angeles' for 'PDT/-7.0/DST' instead in /home/catbu0/public_html/sites/all/modules/ubercart/uc_cart/uc_cart.module on line 288.
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 'America/Los_Angeles' for 'PDT/-7.0/DST' instead in /home/catbu0/public_html/sites/all/modules/ubercart/uc_cart/uc_cart.module on line 295.

The same happened a while ago and I was told I needed to update the Drupal version of the website. That was done and the message went away but now it has come back.