Dates and how to set their hours in ms sql server?

hello this is my store procedure and i want to set the hour of the first date to be 00:00:00 and the hour of the second date to be 23:59:59 but i don't know how to do this,so please HELP me

CREATE PROCEDURE dbo.Proba
@date1 smalldatetime ,
@date2 smalldatetime,
@s float output
AS
Select @s=sum(Pay) from RKO where Dates between @date1 AND @date2
GO

even if the procedure is called with some non-zero time component?

the trick here might be something like

... where Dates >= @date1 AND Dates < dateadd(d,1,@date2)