I have now had some time to check the math for the formula.

The short story is that if the anchor date is a leapyear the algorithm fails. If you change the anchor date to 20000101 or 18960101, the algorithm will give wrong result. That gives us non-leapyears to work with.

Any non-leapyear starting not starting with monday will faill too.

First day of year is Monday

+-----+-----+

| Yes | No |

|-----+-----+

Yes | F | F |

+-----+-----+

No | W | F |

+-----+-----+

Leap year

Accidentally, the date 00010101 will work as it is a non-leapyear starting with monday, and so will 17530101 and 19000101.