Error message

• Warning: date_timezone_set() expects parameter 1 to be DateTime, boolean given in format_date() (line 2089 of /home/y11y1258/public_html/includes/common.inc).
• Warning: date_format() expects parameter 1 to be DateTimeInterface, boolean given in format_date() (line 2099 of /home/y11y1258/public_html/includes/common.inc).