How to do math when question is the last one in your survey?

Assuming you are using JS to do the date calculation and storing it in an embedded data field, you can use the addOnPageSubmit() function. You'll need to get the date from the current page directly from the input field rather than pipe it.

TomG I am close but not quite there. If you can help me get over this hurdle that would be great.
I am using flatpicker, I've loaded all the links for that and jsmoment in my survey header.
For the first segment of my survey, I need to do math between dates that are entered in two different questions on one screen. I am finding that I need to have my Javascript on all one question to make it work, which means I need to be able to hard code one of the two dates and not use this.questionId. I've been doing this in steps instead of one easy line of code to check that I am reading in my variables correctly (and QID5 is coming in properly!). Right now I have the following Javascript on QID5...
//
Qualtrics.SurveyEngine.addOnPageSubmit(function(){
var textDateID5 = moment(jQuery("#"+this.questionId+" .InputText").val());
Qualtrics.SurveyEngine.setEmbeddedData("ID5date", textDateID5);
var textDateID4 = moment(jQuery("#QR~QID4. InputText").val());
Qualtrics.SurveyEngine.setEmbeddedData("ID4date", textDateID4);
var textidage = moment(textDateID5).diff(moment(textDateID4),'years');
  Qualtrics.SurveyEngine.setEmbeddedData("textIDage", textidage);
});
//
From my testing I know that I am breaking/erroring out on pulling in the value for QID4. Any help you can give me on reading in a value without using this.questionId is greatly appreciated.
Ultimately I have to calculate age and differences in days depending on dates I am comparing. I am sure I can do that by changing the second to last line to end with 'days' instead of 'years'. But reading in the value for QID4 is the last bump I anticipate running in to and need help with.
Thank you!