Date object value only within own scope?

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Javascript

Hi, I have a javascript in which I would have 2 date object defined as new Date(). In the middle of the javascript, I would have some logic processing the 2 date object and modify their value accordingly. Finally, I would have a simple subtraction to retrieve the difference the 2 date object.
My codes as follow:

var dateObj = new Date();
var dateObjNew = new Date();

var i = 0;
while(i < 1){
//some logic
anything.then(function(data){
  dateObj.setTime(dateObj.getTime() + 300000); //300000 is in milliseconds
  //means I would add 300k milliseconds into the Date object
});

anything2.then(function(data){
  dateObjNew.setTime(dateObjNew.getTime() + 100000); //add 100000 milliseconds here
});
alert('Remaining Time: '+ (dateObj - dateObjNew)); //here the alert will show 'Remaining Time: 0' because the 2 date objects value has been defaulted back to new Date() where they do not take the latest modified values.
i++;
}

What I have tried:

1. Tried to add getTime() in the final alert but results still is 0.
2. Search online but articles browsed seems not to have this kind of issue.
3. Tried add alert within the 2 .then(function(data)) and the date objects have been updated with the intended milliseconds. E.g. DateTime now been added 300000 milliseconds.