I’ trying to get the sum of AmtDue between two dates. If I use a constant text date-range the Execution Time is 0. If use a variable, which I need to use; it takes 18 seconds. What is causing this?

declare
@BegDate		datetime,
@EndDate		datetime,
@BegDateChar		char(19),
@EndDateChar		char(19)

set @BegDate	= convert(datetime,'03/04/2008 00:00:00')
set @EndDate	= convert(datetime,'03/05/2008 23:59:59')

set @BegDateChar= convert(char,@BegDate)
set @EndDateChar= convert(char,@EndDate)

select 
@BegDate	as BegDate,
@EndDate	as EndDate,
@BegDateChar	as BegDateChar,
@EndDateChar	as EndDateChar

--Query-1 (Execution Time: 00:00:00) 
select sum(AmtDue) as AmtDue
	from invfl with (nolock) 	
	where invdate between '03/04/2008 00:00:00' and '03/05/2008 23:59:59'

--Query-2 (Execution Time: 00:00:18) 
select sum(AmtDue) as AmtDue
	from invfl with (nolock) 	
	where InvDate between @BegDateChar	and @EndDateChar

--Query-3 (Execution Time: 00:00:18) 
select sum(AmtDue) as AmtDue
	from invfl with (nolock) 	
	where InvDate between @BegDate		and @EndDate 

Can you please tell me what the correct syntax is?