Date format subtraction issue with GlideDateTime

Hello,

I have a field within a scoped application for my customer service team which calculates "Resolution Days." I am currently using a business rule to calculate this by getting the "Inquiry Date" and "Resolution Date" fields and subtracting the two, then converting this to days from milliseconds. This was working fine until I added my European user base. It seems the subtraction is having trouble subtracting dates in dd-MM-yyyy format and coming up with incorrect results, as the system default is MM-dd-yyyy.

I tried to change this by setting the GlideDateTime's format with setDisplayValue. However, this is throwing an error:
exception: org.mozilla.javascript.EvaluatorException: Can't find method com.glide.glideobject.GlideDateTime.subtract(undefined,com.glide.glideobject.GlideDateTime).

Below is my business rule:

(function executeRule(current, previous /*null when async*/) {
	//store inquiry and resolution dates
	var inquiryDate = current.getDisplayValue('u_inquiry_date');
	var resolutionDate = current.getDisplayValue('u_resolution_date');


	if(inquiryDate != ''){
		//starts new GlideDateTime object using inquiryDate
		var startDate = new GlideDateTime (inquiryDate);
		//line which is not working 
	//	startDate = startDate.setDisplayValue(inquiryDate, "MM-dd-yyyy HH:mm:ss");
		var endDate = new GlideDateTime(gs.nowDateTime());
		if(current.getValue('u_state') != 'Open' && resolutionDate != ''){
			endDate = new GlideDateTime(resolutionDate);
	//		endDate = endDate.setDisplayValue(resolutionDate, "MM-dd-yyyy HH:mm:ss");
		}
		var dur = GlideDateTime.subtract(startDate, endDate);
		//convert the answer from milliseconds to days
		var days = dur.getNumericValue() /  (1000*60*60*24);
		if(days < 1){
			days = 1;
		}
		current.u_resolution_time = days;
	}
	else{
		current.u_resolution_time = 0;
	}

})(current, previous);

I likely think this is just me not understanding the syntax of using the setDisplayValue() method and how it works. I have read the developer documentation on setDisplayValue, but it is not sinking in for me. Can anyone offer assistance on when I should be converting the date's format / how this is achieved or just general tips for this? Thanks!