Autonumbering Base on other Column Data

Hello Guys,

I hope you can help me on my problem.

I need to have a formula that will result on the following output:

A B
Yes 2013-01-0001
Yes 2013-01-0002
No N/A
Yes 2013-02-003
No N/A
No N/A
Yes 2013-03-004

If a user enter yes, Column B will have an incremental number based on date. But if user, enter no, the column B will just display N/A.

Thanks