I had the same problem. My timezone settings match. I noticed that the problem was only with the labels. If I marked an object to not be available on Sunday, it would show on the calendar as not available on Saturday. When you clicked in the Friday box, a Saturday date was entered. In other words, it worked. Only the labels were wrong.
I edited views/subject/tmpl/default_calendar_weekly.php to fix the problem. I know this is not really the right place for the fix. It looks like this file gets the date from helpers/booking.php in the admin components area.
Because this is not really the right way to fix this and the code I used is ugly, I was reluctant to post this. But, this has worked for me and may help others with this problem.
Around line 70 of the file
<!-- <h2><?php echo AHtml::date($firstDay->date, ADATE_FORMAT_NICE); ?></h2> -->
<h2><?php echo date( 'D d M', strtotime ( '+1 day' , strtotime ( AHtml::date($firstDay->date, ADATE_FORMAT_NICE) ) ) ); ?></h2>
Hope this helps someone else.