This is a JavaScript issue rather than postman.
Search Google for JavaScript date conversion.
Postman does have the moment library available, so I would look that up as well.
To do a comparison you need both dates in the same format. I would recommend using the moment library to convert the dates to a number using the format yyyymmdd (no hyphens or spaces) as technically it’s just a number comparison after this.
It might be worth looking at the following which is another topic related to dates.
How to verify if ISO date on a endDate endPoint is not expired - Just getting started - Postman