Take some base date which is the 31st of some month e.g. '20011231'. Then use the
following procedure (I have given 3 identical examples below, only the @dt value differs).
declare @dt datetime;
set @dt = '20140312'
SELECT DATEADD(month, DATEDIFF(month, '20011231', @dt), '20011231');
set @dt = '20140208'
SELECT DATEADD(month, DATEDIFF(month, '20011231', @dt), '20011231');
set @dt = '20140405'
SELECT DATEADD(month, DATEDIFF(month, '20011231', @dt), '20011231');