Have you set the correct time zone for the site? See http://en.support.wordpress.com/settings/general-settings/#timezone
If you have already done that, and you’re still having problems, then you can use a time format with an explicit time zone:
12/Oct/2014:12:34:56 -0500
For a list of other time formats recognized by PHP’s strtotime() function, please see http://php.net/manual/en/datetime.formats.php