This is how I calculate First/Last day of any given month and prior year corresponding dates.

Leap Year

DECLARE
  @Date DateTime,
  @FirstDay DateTime,
  @LastDay DateTime,
  @PriorFirstDay DateTime,
  @PriorLastDay DateTime

SET @Date='2/15/2013'

SELECT @FirstDay = CAST(MONTH(@Date) as varchar(2))+'/1/'+CAST(YEAR(@Date) as varchar(4))
SELECT @LastDay = DATEADD(day, -1, CAST(MONTH(DATEADD(month, 1, @Date)) as varchar(2))
                         + '/1/' + CAST(YEAR(DATEADD(month, 1, @Date)) as varchar(4)))
SELECT @PriorFirstDay = DATEADD(year, -1, @FirstDay)
SELECT @PriorLastDay = DATEADD(day, -1, CAST(MONTH(DATEADD(month, 1, DATEADD(year, -1, @Date))) as varchar(2))
                         + '/1/' + CAST(YEAR(DATEADD(month, 1, DATEADD(year, -1, @Date))) as varchar(4)))
                         
SELECT  @Date AS [Date], 
@FirstDay AS [FirstDay], 
@LastDay AS [LastDay], 
@PriorFirstDay AS [PriorFirstDay],
@PriorLastDay AS [PriorLastDay]