I'm having difficulty with this scenario:  I have a custom button that uses onclick Javascript to create 14 custom object records.  I want each of the custom object records that is created to have a different value in the field Planned Date.  The value should be Created Date + x where x varies across the 14 records. 

Here's the code that isn't working.  (It works fine if I take out the + # and sets Planned Date to the current date for every record.)  I'm trying to add a given number of days to the day on which the record in this batch was created to determine and set the Planned Date of each record: 


var plans = [];

var plandate = new Date();

plans[0] = plandate.setDate(plandate.getDate() + 4);
plans[1] = plandate.setDate(plandate.getDate() + 10);
plans[2] = plandate.setDate(plandate.getDate() + 17);
plans[3] = plandate.setDate(plandate.getDate() + 24);
plans[4] = plandate.setDate(plandate.getDate() + 35);
plans[5] = plandate.setDate(plandate.getDate() + 14);
plans[6] = plandate.setDate(plandate.getDate() + 40) ;
plans[7] = plandate.setDate(plandate.getDate() + 40);
plans[8] = plandate.setDate(plandate.getDate() + 14);
plans[9] = plandate.setDate(plandate.getDate() + 30);
plans[10] = plandate.setDate(plandate.getDate() + 35);
plans[11] = plandate.setDate(plandate.getDate() + 50);
plans[12] = plandate.setDate(plandate.getDate() + 64);
plans[13] = plandate.setDate(plandate.getDate() + 70);

for (var i=0; i<14; i++) {
    var object = new sforce.SObject("Object__c");
...other code...
    object.Planned_Date__c = plans[i];
...other code...