API Project: Timestamp Microservice: return the expected error message?

Tell us what’s happening:
Only the test: It should return the expected error message for an invalid date fails.

But why?

For invalid date the following is returned, what is wrong with it?
{"unix":null,"utc":"Invalid Date"}

Your code so far
https://repl.it/@controlunit/boilerplate-project-timestamp#server.js

Your browser information:

Chromium

Challenge: Timestamp Microservice

Link to the challenge:

Hey there,

did you have a look at the timestamp tests?

What is the exact requirement for the expected error message?
The code produces: {"unix":null,"utc":"Invalid Date"}, why is this incorrect?

It is not clear what the error message is supposed to be?