Calculating Date From Month (sharepoint 2010)

See more:

SharePoint

DateTime

, +

I have a month column (choice column) in my DOCUMENT LIBRARY in the format specified below..
For eg. Jan 2013

I have the following requirements
1. The month dropdown should always have only those months which start from 6 months previous of the current month.
For eg.
If the current month is March 2013 then only the months starting from oct 2012 to march 2013 should be populated in the dropdown

2.There is another column in my document library called 'date' of date format.It is not to be shown in the default view.The date field should automatically be set as the 1st date of the month selected from the month column.
For example.
IF the month selected is March 2013 .The date Should b 1/3/2013. Please Help!!Its urgent.. Any Help is welcome...