Discussion on: How to mock Date with Jest

I just want to point out that both Date and RealDate reference the same object, so when you replace the function Date.now, it's also changing RealDate.now and your global.Date = RealDate at the end is not doing what you think it's doing.

What you should do if you want to keep and reassign the original now function is keep a reference to that function:

const realNow = Date.now

Date.now = () => 1000

Date.now = realNow