Zero is 01 jan 1900 in SQL, so you can use this:
DATEADD(day, @dayval-1,
DATEADD(month, @monthval-1,
DATEADD(year, @yearval-1900, 0)
)
)
Edit, Feb 2018
As the other answer says, since SQL Server 2012 (released after the original answer) we can use DATEFROMPARTS
SELECT DATEFROMPARTS (@yearval, @monthval, @dayval)
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