Problems adding days to a date.

I have a form passing a date in the format of 2007-01-12. I need to add 56 days to that to come up with a begining and ending date to use in a query. Almost everything that I have tried has give me very wierd results.

For example the following date being passed 2007-01-21, to the following code,

      $start_date = $_POST['snapshot_date'];
      $end_date = $start_date + (86400 * 7);
      echo "$end_date";

yields a result of 606807

I have tried using the strtotime() function and it yields similar incorrect results

where am i off point