Okay, here's the stuff:
1. The function was wrong (sorry about that, that is my fault - the part of [Last Test Date]) should have been dteDate.
2. You can't have the controls and field names named the same when doing calculated fields. Access gets confused.
3. You had the function in the wrong control. I needed to be in the NEXT TEST control and not the Frequency control.
4. the function was called incorrectly. You just need to pass the field names in brackets.
5. You had criteria in your query of [monthly] OR [Monthly] which the brackets mean to Access, in that case, to prompt the user for input. If you had wanted to select only the monthly values then it would have been "Monthly" in quotes (and it is case-insensitive).
See attached.