it looks like you put the max date and min date code in the server validation behavor without the opening and closing php tags.
in the min date field, use this code:
<?php echo(date("m/d/Y", mktime(0, 0, 0, date("m") , date("d")-7, date("Y")))); ?>
in the max date field, use this code:
<?php echo(date("m/d/Y")); ?>
make sure you are including the opening php tag:
and the closing php tag: