The code on the top of the page should be:
<?php $writeDate = strtotime($row_Recordsetcustomers['date']); ?>
and to increment I think you are going to have to use:
$writeDate = strtotime('+7 days', date('Y-m-d H:i:s' , $writeDate));
you need to learn how to debug if you are going to try to write php code by hand. Have you tried adding die() and echo() statements to your code so you can check when and where things are going wrong? Are you looking for code examples on php.net and trying to adjust lines of code when there are errors?
I like helping, but debugging and writing custom code go hand in hand. You should learn debugging now before wasting this much time in the future. If you knew how to debug this problem would have been solved in the first hour of working on it and you wouldn't be worried about your deadline.