Always handle dates as dates, not strings, not numbers, no exceptions.
And when you write SQL, use the correct syntax. The text expression for a date value must be like mm/dd/yyyy or yyyy/mm/dd.
You did use dd/mm/yyyy, thus your date was read as 2018-02-08. So use:
CDate([a.acdate]) <= #2018/08/02#
and your query will run as expected.