How to write customer exit for the variable

Hi Experts,

I have a requirement to create the variable, the scenaria is like this..

I need to create the variable which gives the period/year values,if yours enters the values 05.2007 then the variable should return the first monthe the year i.e.01.2007.

I hope it can be done by writing the customer exit..but iam unware how to achieve this.

Please explain me step by step and cope for customer exit to done this.

Points will be awarded

Suraj.