Help with calculated column formula- combine strings and convert to date

In my list (SharePoint Server 2013) I have:

'Invoiced Month' column, type of choice (strings with names of months- January, February, March..., December)

'Year' column, single line of text (e.g. 2012, 2013, 2014)

1. I need to create a calculated column which will return combined value of the columns above, but in DATE format e.g. 'Sep-2013' or '01-Sep-2013'.

I then use that newly created calculated column to do this: http://iwillsharemypoint.blogspot.in/2012/03/sharepoint-list-view-of-current-month.html

I am rubbish with formulas, can I have some help with my problem please?