Public Function FUN_BRONb_DATA_VIBOR() As Variant
If IsDate(Form_BRONb_FRM!BRONb_DATA_VIBOR) = False Then
 FUN_BRONb_DATA_VIBOR = Null
Else
 FUN_BRONb_DATA_VIBOR = Nz(Form_BRONb_FRM!BRONb_DATA_VIBOR)
End If
'MESS Nz(FUN_BRONb_DATA_VIBOR)
End Function

SELECT Таблица1.фио
FROM Таблица1
WHERE IIf(FUN_BRONb_DATA_VIBOR() Is Null,Not IsNull(дата) or Таблица1.дата is null,FUN_BRONb_DATA_VIBOR()=Таблица1.дата);