Help with Case if statement

I have an if then else statement i used in another application that I am trying to replicate in CRM on Demand witht he Opportunity."Close Date" field.

Can anyone help me translate this? I put the Opportunity."Close Date" field where it woudl be if theis statement worked

if (Opportunity."Close Date" >= _first_of_month(current_date) and Opportunity."Close Date" <= _last_of_month(current_date)) then ('30 Day') else if (Opportunity."Close Date" >= _first_of_month(_add_months(current_date,1)) and Opportunity."Close Date" <= _last_of_month(_add_months(current_date,1))) then ('60 Day') else if (Opportunity."Close Date" >= _first_of_month(_add_months(current_date,2)) and Opportunity."Close Date" <= _last_of_month(_add_months(current_date,2))) then ('90 Day') else ('90+ Day')