How do you do two date calculations in one SQL Statement?
Given a date:

DECLARE @date datetime = CONVERT(date, GETDATE());


--The beginning of this month:

SELECT DATEADD(DAY, 1-DAY(GETDATE()), @date);


-- The beginning of last month - just subtract a month:

SELECT DATEADD(MONTH, -1, 
  DATEADD(DAY, 1-DAY(GETDATE()), @date));


-- Then to get the 17th of last month, just add 16 days:

SELECT DATEADD(DAY, 16, DATEADD(MONTH, -1, 
  DATEADD(DAY, 1-DAY(GETDATE()), @date)));


That will work on 2008 and above. On 2012 you can write it a little shorter:

DECLARE @date datetime = DATEADD(MONTH, -1, GETDATE());

SELECT DATEFROMPARTS(YEAR(@date), MONTH(@date), 17);


If you are doing this for searching, though, and not output, I highly recommend adjusting the date range parameter values, not applying DATEADD() operations data in the loanFirstPmtDate column.