Datatype mismatch in criteria

[Snoopy33]

I have a query that builds a date from a text field with the date
imput as yyyymmdd in the query, so i have to piece it together in my
expression as follows:

exp: FormatNumber(DateDiff("d",DateSerial(Left([package activity date],
4),Left(Right([package activity date],4),2),Right([package activity
date],2)),Now())))
if i don't use a criteria, it works fine. when i try to use the
criteria <7 is when i get the error. It will show the correct records
for a few seconds prior to getting the error, but then i get an error
"datatype mismatch in criteria".

I've tried it with and without the formatnumber function, with and
without the dateserial function (rearranging the expression) and still
get the error.

Any help?