Time zone error

User view of appointment form gives this error:
Warning: date() [function.date]: 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/Chicago’ for ‘CDT/-5.0/DST’ instead in …calendar_form.php
Admin view of appointment form doesn’t have the same error.
I don’t see a timezone string in PHP.ini
http://www.reddesertviolin.com/RDVnew/adult-violin-students-your-time-is-now/#

https://ww.wp.xz.cn/plugins/appointment-calendar/