How to build a "Case"

I need to build a "case style" Calculated_date based on a field that contains a list of values (Frequency). 
Like:
IF "Frequency" is "Annual" then Calculated_Date = Initial_date + 1 year (or +365 days)
IF "Frequency" is "Monthly" then Calculated_Date = Initial_date + 1 month (or +30 days)
IF "Frequency" is "Daily" then Calculated_Date  Initial_date + 1 day.

How can I do that ?

Thanks for your help!