declare @datetime datetime;
set @datetime = getdate();
select @datetime;
select dateadd(year,datediff(year,0,@datetime),0);
select dateadd(month,datediff(month,0,@datetime),0);
select dateadd(day,datediff(day,0,@datetime),0);
select dateadd(hour,datediff(hour,0,@datetime),0);
select dateadd(minute,datediff(minute,0,@datetime),0);
select dateadd(second,datediff(second,'2000-01-01',@datetime),'2000-01-01');

create function dbo.DATEFLOOR (
  @seed datetime
, @part varchar(2)
)
returns datetime
as
begin

/*
Sample Usage (uses standard dateparts): 
select 'second', dbo.datefloor(getdate(),'ss') union all
select 'minute', dbo.datefloor(getdate(),'mi') union all 
select 'hour' , dbo.datefloor(getdate(),'hh') union all
select 'day' , dbo.datefloor(getdate(),'dd') union all
select 'month' , dbo.datefloor(getdate(),'mm') union all
select 'year' , dbo.datefloor(getdate(),'yy') 
*/

select @retDate = case
when @part = 'ss' then dateadd(second,datediff(second,'2000-01-01',@seed),'2000-01-01')
when @part = 'mi' then dateadd(minute,datediff(minute,0,@seed),0)
when @part = 'hh' then dateadd(hour,datediff(hour,0,@seed),0)
when @part = 'dd' then dateadd(day,datediff(day,0,@seed),0)
when @part = 'mm' then dateadd(month,datediff(month,0,@seed),0)
when @part = 'yy' then dateadd(year,datediff(year,0,@seed),0)
end

return @retDate
end