Thanks Sergio and Michal,
I have finally made a calculated column with this expression: format("ERDAT", 'MM.YYYY')
But it's not the solution that I was looking for. Because in this way I can't use that field to compare between two months.
Our application is formatting the data dynamically according to its data type in the front-side.
So I would like to create a custom data type for "CALMONTH" to keep this field in YYYYMM format in views and tables and to show on client-side in another format that is more appropiate, but I could not find a way to do that.
Thanks!