Sample code for finding difference between 2 dates. You can make changes as per your req.

var grTask = new GlideRecord('sc_req_item');
grTask.addQuery('sys_id','5eec52291b3e3300364d32a3cc4bcb08');
grTask.query();
if(grTask.next()){
gs.print('Number'+grTask.number+"|| Created ON: "+grTask.due_date);
var gdt1 = new GlideDateTime(grTask.due_date);
gs.print("CreatedDate: "+gdt1.getDate());

var gdt = new GlideDateTime('2019-07-22 03:56:25');
gs.print("GDT:"+gdt.getDate());

var startDate = gdt1.getDate(); // Replace u_start_date with your field name of Start Date
var currentDate = gdt.getDate();

var diffSeconds = gs.dateDiff(currentDate.getDisplayValue(),startDate.getDisplayValue(), true);
var daysIn = (diffSeconds/(60*60*24));

gs.print(daysIn );
}

Please mark as Correct Answer/Helpful, if applicable.
Thanks!
Abhishek Gardade