declare @dt char(10)

select @dt = '31/01/2010'

--This will always work
select convert(smalldatetime, @dt, 103)

--This might work and might fail 
select cast (@dt as smalldatetime)
go

declare @dt char(10)

select @dt = '01/31/2010'

--This will always work
select convert(smalldatetime, @dt, 101)

--This might work and might fail 
select cast (@dt as smalldatetime)