Frustration with datePicker dpSetSelected

Frustration with datePicker dpSetSelected

Hi, I'm trying to use the datepicker plugin to allow users to select start and end dates from a range of values.  The code I'm using to set limits is as follows;
 
  1. var startdate = new Date().addDays(-56).asString();
     var curdate = new Date().asString();
     var enddate = (new Date().addYears(1).asString());


  2.  $('#dateFrom_id').dpSetSelected(startdate);
     $('#dateFrom_id').datePicker().val(startdate).trigger('change');
     $('#dateTo_id').datePicker().val(curdate).trigger('change');
     $('.date-pick').dpSetEndDate(enddate);
     $('.date-pick').dpSetStartDate(new Date().addYears(-10).asString());




 
As you can tell from the code, the IDs for the relevant input fields are 'dateFrom_id' and 'dateTo_id'.  The effect I'm trying to achieve here is to set the dateFrom_id field to a date 8 weeks in the past, and set the dateTo_id field to the current date.  The input field values are being set correctly, the date 8 weeks past is just what I'm trying to achieve, but when I open the datePicker for the dateFrom_id field, it is starting on the current date instead of the startdate.  I've tried a number of variations of the code to get what I'm looking for, and I've checked the documentation, which seems to my interpretation to indicate that simply using the dpSetSelected method should achieve my goal.
 
But it's not working.
 
If anyone can point out what I'm doing wrong, and how I might fix it, I'd really appreciate it.  BTW, I've also tried the format of
 
  1. $('#dateFrom_id').datePicker().dpSetSelected(startdate);
 
and that, too, has failed.  Thanks in advance for any help.