This relationship is probably already known about but I haven't been able to find anything written about it.

Suppose A and B are days of the year.

If A and B are 99 days apart, then B and A are also 999 - 3 days apart (and +1 if it spans a leap year).

Therefore, you can choose any day of the year, add 99 + 999 - 3 days, and you will get the exact same day just 3 years later.

For example,

Oct 22/20 .... + 99 days .... Jan 29/21 .... + 999 - 3 days .... Oct 22/23.

Leap year example,

Oct 22/21 .... + 99 days .... Jan 29/22 .... + 999 - 3 + 1 days .... Oct 22/24 (LY).

Something about having all 9s and only needing to subtract 3, which mirrors the 3-year duration, is interesting.