In order to change the Date/Time format, you need to change it on your system. Access (all apps) use the system time format, so perhaps this is effecting your ability to change this parameter.
If you are using a form field to hold the Date and Time (I don't know if it is one field or two) then in the query design, in the criteria for the date (and time) field(s), put this code:
When the query is run, it will look for the criteria the user has selected on your form.
You may want to put a check on the button that calls the query or the report to see if the field is populated, otherwise the user will get an error.