How to get a Jest custom matcher working in typescript?

The other question and answer you linked to were correct, and you can also find a very succinct example for how to extend jest in this github comment on react-testing-library.

To implement their solution for your code, just change:

declare global {
  namespace jest {
    interface MomentMatchers extends Matchers<moment.Moment> {
      toBeSameMoment: (expected: moment.Moment) => CustomMatcherResult;
    }
  }
}

To:

declare global {
  namespace jest {
    interface Matchers<R> {
      toBeSameMoment(expected: moment.Moment): CustomMatcherResult;
    }
  }
}