How to create a ticket numbering where it´s cleared all year beginning

Hi,

I need to create an attribute to store ticket number set automaticaly, and this number need to be cleared at every year beginning (on January, 1st).

This number has the following format: SSSnnnn/yyyy, where nnnn is a counter, and yyyy is the current year. For example, it may be SSS0099/2013 this year, but it need to start at SSS0001/2014 next year.

I created a calculation where the number is calculated using Id attribute, but this Id is continuously increased. So, if it was possible to reset this Id number, I would have the problem solved.

Thanks