Assume I have a dataframe with one column of class “Date” in R:

       Date      Sales      Type   
1 2012-03-22      22          A      
2 2012-03-22      18          B      
3 2012-04-11      20          C      
4 2012-05-03      26          A      
5 2012-05-03      15          C     
6 2012-06-12      29          A      
7 2012-07-15      22          B      

I want to create dummy variables for certain groupings of months in order to run various linear and glm models. For instance, if I form 3 groups consisting of months {3}, {5} and {6,7} respectively, I want 3 dummy variables, call them Dummy1, Dummy2 and Dummy3 respectively to get the following table:

       Date      Sales      Type   Dummy1  Dummy2  Dummy3
1 2012-03-22      22          A      1       0       0
2 2012-03-22      18          B      1       0       0
3 2012-04-11      20          C      0       0       0
4 2012-05-03      26          A      0       1       0
5 2012-05-03      15          C      0       1       0
6 2012-06-12      29          A      0       0       1
7 2012-07-15      22          B      0       0       1  

How can I achieve this?