Why can't I use a variable to pass a date to this sub to set parameter values?

In case it's relevant, this is the full code of the sub I used with the post above as basis.

Function SetRecordset(frm As Form, q As String, ParamArray aParam() As Variant)

1590      If gcfHandleErrors Then On Error GoTo Err_Handler

          Dim db As DAO.Database

          Dim qdf As DAO.QueryDef

          Dim rs As DAO.Recordset

          Dim i As Integer, c As Integer

1600      c = UBound(aParam())

1610      Debug.Print frm.Name

1620      Set db = CurrentDb

1630      Set qdf = db.QueryDefs(q)

1640      Select Case q

          Case "qryNoExamsOOSAssigned"

1650      Case Else

1660          For i = 0 To c

1670              qdf.Parameters(aParam(i)) = aParam(i + 1)

1680              i = i + 1

1690          Next i

1700      End Select

1710      Set rs = qdf.OpenRecordset(dbOpenDynaset)

1720      Set frm.Recordset = rs

Exit_Handler:

1730      Exit Function

Err_Handler:

1740      clsErrorHandler.HandleError "Exam", "SetRecordset", True

1750      Resume Exit_Handler

End Function

Just in case it matters, since we're talking about dates, what is your Regional Settings?

As a general observation, when establishing a recordset object one should wherever possible concatenate the values into the string expression for the SQL statement, rather than assign the values to parameters.  All that's then necessary is to ensure that the values concatenated into the string expression are correctly delimited, where necessary for the data type in question.  In the case of strings I would recommend representing the literal quotes delimiters by means of contiguous pairs of quotes characters, i.e. """" rather than "'".  It is more likely that a string expression will contain an apostrophe than a double quotes character, particularly in the case of personal names.  Date literals must be delimited by the # character, and be in US or an otherwise internationally unambiguous  format such as the ISO standard for date notation of YYYY-MM-DD hh mm ss (note that the standard uses case to differentiate months and minutes, whereas Access uses n for the latter). 

In the case of a saved querydef object which might include parameters, these should be references to an accessible object, usually a control in an open form.  The code can then loop through the query's Parameters collection and evaluate each parameter's Name property with the Eval function.  There is no need to know the names of the parameters, or whether the query has parameters or not.

Standard US settings.

So, you're saying I should be using the SQL and not the saved queries for these forms? That seems annoying if I'm using the same query in multiple forms and have to make an edit. But I guess there's the argument that I probably won't be changing these very much, if ever, once I get it all working...

I'm afraid I'm just not advanced enough to really understand the second paragraph there. I have no grasp of how I'd assign values to something that may or may not exist, and I don't give the name of it if it does exist. Maybe when I do another revision of this project in the future I will be.