Doing Math with Date Values - google-apps-script

Question

I am trying to use the difference between two dates in my Google Sheet to determine the number of rows down that the originRange will be copied.
I'm getting an error 'Cannot convert NaN to (class)' so I'm assuming I need to change the format of the date in some part of the formula but I am completely stuck.
The dates in the Google Sheet are formatted "10/26/2016" & "12/31/2016" if that helps at all.
function fillNewReport () {
var startDate = responseSheet.getRange('C2');
var endDate = responseSheet.getRange('G2');
var dateDiff = (endDate - startDate) + 1;
var newReport = ss.getSheetByName('Copy of Form Responses 1');
var originRange = newReport.getRange(2,2,1,22);
var newRange = newReport.getRange(2,2,dateDiff,22);
originRange.copyTo(newRange);
}
Any help with this will be greatly appreciated.
Thank you,
EDIT: Part 2
I think I've got the right formatting now, as I'm no longer getting an error. But the formula is not responding. No rows are being copied. Any suggestions?
function fillNewReport () {
var startDateValue = responseSheet.getRange('C2').getValue();
var endDateValue = responseSheet.getRange('G2').getValue();
var startDateMilli = new Date(startDateValue);
var endDateMilli = new Date(endDateValue);
var startDate = startDateMilli.getDate();
var endDate = endDateMilli.getDate();
var dateDiff = (endDate - startDate) + 1;
var newReport = ss.getSheetByName('Copy of Form Responses 1');
var originRange = newReport.getRange(2,2,1,22);
var newRange = newReport.getRange(2,2,dateDiff,22);
originRange.copyTo(newRange);
}

Answer 1

In your code startDate and endDate are ranges, not values !
So first thing is to change that using getValue().
You will then get date objects that wont behave as you expect because unlike days in spreadsheets javascript date objects are more complex objects, they have date and time properties and have a native value of the number of milliseconds since the reference date...
Anyway, all you have to do is using the date methods described in all javascript reference on the internet, ask Google for "JS DATE"
Then do the math you need, this part will be easy I think.
edit
As I wrote in the comments, I'm not sure I understand what result you want to get...
I tried a test changing a few details and I get a result... you tell me if that's what you wanted.
function fillNewReport () {
var startDateValue = responseSheet.getRange('C2').getValue();
var endDateValue = responseSheet.getRange('G2').getValue();
var startDateMilli = new Date(startDateValue);
var endDateMilli = new Date(endDateValue);
Logger.log('startDateMilli = '+startDateMilli+' endDateMilli = '+endDateMilli);
var startDate = startDateMilli.getDate();
var endDate = endDateMilli.getDate();
var dateDiff = (endDate - startDate) + 1;
Logger.log('dateDiff = '+dateDiff);
var newReport = ss.getSheetByName('Copy of Form Responses 1');
var originRange = newReport.getRange(2,2,1,22);
var newRange = newReport.getRange(2,2,dateDiff,22);
originRange.copyTo(newRange);
}
or like this to copy only one row
...
var originValues = newReport.getRange(2,2,1,22).getValues();
Logger.log('originValues = '+originValues);
var newRange = newReport.getRange(2+dateDiff,2,1,22); // copies to the other sheet but dateDiff rows underneath...
newRange.setValues(originValues);
}