What have you tried ? Do you know anything about SQL at all ? Have you considered that asking random strangers to do your work for you is not a great long term business plan ? As I understand this question, it seems trivial to me. Sure, these forums are here to help people, but you know, it would make us feel better if you showed some signs of have a basic understanding of the issue, and of being willing to try to work it out for yourself, too. As it stands, I am not sure you'd understand the answer if we gave it.
I googled and found some SQL code that can take a day, month and year and make a date time using the dateadd function. I wrote an article on how to use google, if you don't know how.
declare @d datetime;
set @d = dateadd(year,(@Year - 1800),'1/1/1800');
set @d = dateadd(month,@Month - 1,@d);
return dateadd(day,@Day - 1,@d)