Did you mean 2021 or 2020?
If you meant 2021, then you get the result for “2020-12-31T16:00:00.000Z” because your server timezone and database zone differ.
Take a look at the timezone setting:
Also, you should configure the timezone on your server in order to get identical results.
Let’s your timezone from server differs with 12 hours from database, them if you send the request from server with 2020-01-01T00:00:00, then in db it will search 2019-12-31T12:00:00.