21st: +1 0th, +1 1st, +1 6th
22nd: +1 2nd, +1 7th
23rd: +1 3rd, +1 8th
24th: +1 4th, +1 9th
25th: +1 5th
26th: +1 6th
27th: +1 7th
28th: +1 8th
29th: +1 0th, +1 1st, +1 2nd, +1 9th
30th: +1 3rd, +1 4th, +1 5th
Blech. It doesn't appear to follow a pattern. I would seriously consider trying to...