Does the health app calculate average annual steps on a calendar year basis or on a rolling year basis?

I was just wondering whether the health app calculates annual average steps on a calendar basis, (i.e. 1st January till present) or a rolling year basis(example: 26th July 2019 - 26th July 2020). If anyone with the knowledge from the Apple team or the community could please tell me the answer to this question then I would be most grateful.