The code looks good.
Can you post an example of an incorrectly converted date?
Also, although you mention converting the number to a date, you didn't indicate if you were storing the information in a 'date' field.
This seems to work fine:
Mid$(Format(21703 ,"000000"

,1,2) & "/" & Mid$(Format(21703 ,"000000"

,3,2) & "/" & Mid$(Format(21703 ,"000000"

,5,2)
as does this:
cdate(Mid$(Format(21703 ,"000000"

,1,2) & "/" & Mid$(Format(21703 ,"000000"

,3,2) & "/" & Mid$(Format(21703 ,"000000"

,5,2))
you arent't by any chance storing dates as D/M/Y are you?
If so, the expression needs to be changed as follows:
Mid$(Format(21703 ,"000000"

,3,2) & "/" & Mid$(Format(21703 ,"000000"

,1,2) & "/" & Mid$(Format(21703 ,"000000"

,5,2)