Half month rule

Dear SAP Guru

MY US books require Mid Month rule for depreciation. We are using year dependent, non-calendar month Fiscal year variant for or Financial accounting. I defined 24 month Fiscal year variant and in " Define Calendar Assignment" I defined for period control Z01 periods as below

Year Month Days period

2007 01 15 1

2007 01 31 2

2007 02 14 3

2007 02 28 4

-

-

2007 12 15 23

2007 12 31 31

After this I assigned this period control NT to my Dep Key, but noow when i am creating the asset, i get an error message which is as below:

" YOU CANNOT DETERMINE THE PERIOD FOR RULE Z01 to 04/20/2007"

Please advise where I am going wrong

Thanks