Your column is a character column, not a JMP numeric date column. To create a column that has just the month part of the character string, use the formula
substr( :"Month-Year"n, 1, 3 );
The structure I used gives the same results as :Name("Month-Year), but the structure I used is the supported structure moving forward
P.S. In the future when you provide an image of a data table, please provide the left hand panels of the data table. They provide more detail about the columns.
![table.PNG table.PNG](https://community.jmp.com/t5/image/serverpage/image-id/32505iE1C76011D1CFA011/image-size/large?v=v2&px=999)
Jim