Combine aggregated weeknumber data and month data to date

[JGroot]

I have a file with aggregated weekdata and monthdata and I want to combine those two columns into one date data column.

See attachment Column A and B.

Week numbers are sometimes divided over 2 months. So I have a row where weeknumber 5 is in January and weeknumber 5 data is in February.

In the 1st case I want to have a result that is 29/1/2012 and the 2nd would be 1/2/2012.

It would be great if someone can help me with the right formula!