WordPress: Warnings with strtotime()

How to solve the following WordPress warning:

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.

Just add the code below before the first time the above strtotime() function is used in this file /wp-includes/functions.php

date_default_timezone_set(‘UTC’);