Summarize Any Day Of The Year In Numerical Terms

I would like to create a code that would summarize any day of the year in numerical terms.

The formula for coding "days" is as follows:

Coding Month value + current day

Where "coding month" was a combination of the year and month.

The year can be summarized as a fixed value:

2007 yr = 4
2008 = 5
2009 = 6

Ignore anything prior to 2007 and anything after 2009.

The month is simply a SINGLE digit to represent the month and where
January=1, feb=2, march=3......Sept=9. Once you hit October, you have to reduce the values to a single digit, so October = 10 = 1+0=1, November =11=1+1 =2, December = 12 = 1+2=3

Therefore, May 2007 = 4 ( 2007 ) + 5 = 9
June 2007 = 4 + 6 = 10 = 1+0 = 1


Now, comes the addition of the individual day to the mix. If you wanted to find the value for May 23, 2007

You know that May 2007 is 5+4 = 9 so adding the 23rd day is 2+3=5
therefore 9 + 5 = 14. Right?

Not really...

Before reducing this final number down further, there are some double digit numbers that DO NOT get reduced further. They are:

a. 13,14,16,19
b. 11,22,33,44,55,66,77,88,99

These numbers have to be left as is. So, today, May 23, 2007 would be left as a "14"

Two more examples:

May 24, 2007 = 5+4+ 24 = 5+4+2+4 = 15 = 6
July 17, 2007 = 7 + 4 + 17 = 7+4+1+7 = 19 = left as 19

Final thought - It would be helpful, though not necessary, to have a notation besides these double digits. If series "a" could have a "KD" next to it and series "b" could have a "MN" next to it,