Based on col dates create col of dates for the first day of that same month and year

I have a column of dates say in col B. In col-A i want the following:

ColA ** ColB
1/1/11 ** 1/14/11
1/1/11 ** 1/28/11
2/1/11 ** 2/4/11
1/1/11 ** 1/12/11
3/1/11 ** 3/14/11


Basically, i have a col dates in B.....i want the first day of that month/year in col-A.

I am enclosing a file showing what i want. I can do it in excel but not sure how to "efficiently" do it in vba.