SQL create a DateTime value from Year and Quarter

Question

I know the year and the quarter (e.g. "2010" and "4") for a schedule-related milestone and I want to select/create a datetime from it. There are a number of nifty ways to identify the quarter with formats ("qq") of a particular date, but not to go the other way around (or are there?). This is with t-sql / SQL Server.

Note: the datetime should be for the last day of that quarter.

UPDATE: Here is the solution that I ended up using courtesy of gbn, with AaronLS's variable names and then shortened-and-sweetened with Frank Kalis' suggestion :-) It was important to test for all 4 quarters to make sure the year is handled properly. Thanks to everyone who answered!

DECLARE @TheQuarter INT
DECLARE @theYear INT
-- Note: qq = q = quarter for the datepart
SET @TheQuarter = 1
SET @TheYear = 2011
SELECT DATEADD(YEAR, @TheYear-1900, DATEADD(qq, @TheQuarter, -1))
-- 2011-03-31 00:00:00.000

SET @TheQuarter = 2
SET @TheYear = 2011
SELECT DATEADD(YEAR, @TheYear-1900, DATEADD(qq, @TheQuarter, -1))
-- 2011-06-30 00:00:00.000

SET @TheQuarter = 3
SET @TheYear = 2011
SELECT DATEADD(YEAR, @TheYear-1900, DATEADD(qq, @TheQuarter, -1))
-- 2011-09-30 00:00:00.000

SET @TheQuarter = 4
SET @TheYear = 2011
SELECT DATEADD(YEAR, @TheYear-1900, DATEADD(qq, @TheQuarter, -1))
-- 2011-12-31 00:00:00.000

Here are a few q's that fetch the quarter from the date but not the other way around: Calculate the Last Day in the CURRENT quarter; Calculate the last day of the quarter; Best way to store quarter and year in SQL Server?

Answer 1

Never use strings for datetime conversions: too much to go wrong with formats, language etc.

Keep it in the datetime type...

Select dateadd(day, -1, 
                       dateadd(year, @year-1900,
                                          dateadd(quarter, @qq, 0)
                                     )
             )

Answer 2

Looks like you've already found your solution, but just for the sake of it... If you choose a different base date, you can shorten the whole thing to

SELECT DATEADD(YEAR, @TheYear-1900, DATEADD(qq, @TheQuarter, -1))

Since 0 indicates SQL Server's base date of 01.01.1900 (and the first day of a month), using -1 as base date starts off 1 day earlier and then you already have your last day of a month (and end of a quarter). Then you just need to do the rest of the datetime magic and voilà.