converting date time format in sql

converting date time format in sql

i have a stored procedure like this:
ALTER procedure [dbo].[startandenddtime]
@startdate nvarchar(100)
as begin
declare @date3 nvarchar(100) = cast(CONVERT(varchar(100), @startdate + '
16:59:59', 120) as datetime)
select date3 as startdate
end
If i pass my startdate as 2013-05-08 i am getting out put as : but i want
to get out put as 2013-05-08 16:59:59.000..so how i can convert this
format is this date time i can store in nvarchar varibale