Multi Select For Query

I have copied the following code into my Database however I cannot get it to
work if the field is of a Date type. I can change the varValue data type from
Variant to Date however this then damages all other fields. Any ideas?

Function IsSelectedVar( _
strFormName As String, _
strListBoxName As String, _
varValue As Variant) _
As Boolean
'strFormName is the name of the form
'strListBoxName is the name of the listbox
'varValue is the field to check against the listbox
Dim lbo As ListBox
Dim item As Variant
If IsNumeric(varValue) Then
varValue = Trim(Str(varValue))
End If
Set lbo = Forms(strFormName)(strListBoxName)
For Each item In lbo.ItemsSelected
If lbo.ItemData(item) = varValue Then
IsSelectedVar = True
Exit Function
End If
Next
End Function