Data set parameter with list box

Hello,

I am contacting you because I have a problem with my dataset parameter. What I want to do is for each execution of the report I display a list box to select the current month or the previous one month. For that I created a data set select * from mytable Where MONTH (myDate) = ? and I specified ChoiceMonth in Report Parameters. ChoiceMonth is a Button radio but the values ​​are static, I also tried a dynamic list box but I do not know how I do for the current month and months précedent .

Thank you for your help.