With additions of the blood moon settings, code would store the last blood moon day to determine the next day from your current settings. There was no way to go back. If last was day 14 and BM frequency was 7, then next would be 21. If you set the day back to 1, next was still 21. The change makes it recalculate when you go back in time, so last would be day 0, next 7 in that case.
You can now go forward and/or back repeatedly in time and it recaclulates the best next BM day each time.