...

Dim dadataeff As String
    
    Me.txt_dadataeff.SetFocus
    
        If Me.txt_dadataeff.Text = "" Then
        
            MsgBox "DATA OBBLIGATORIA!", vbCritical, "ATTENZIONE"
        Else
            
            dadataeff = "Qdate.DATA_EFFPARTENZA >= #" & Format$(Me.txt_dadataeff, "mm/dd/yyyy") & "#"
            
        End If

    
    Dim adataeff As String
    
    Me.txt_adataeff.SetFocus
    
        If Me.txt_adataeff.Text = "" Then
        
            MsgBox "DATA OBBLIGATORIA!", vbCritical, "ATTENZIONE"
        Else
            
            adataeff = "Qdate.DATA_EFFPARTENZA <= #" & Format$(Me.txt_adataeff, "mm/dd/yyyy") & "#"
            
        End If

    
    If dadataeff <> "" And adataeff <> "" Then
    
    StrSQL = "SELECT * FROM Qdate" & _
    " WHERE " & dadataeff & " AND " & adataeff & " ORDER BY Qdate.DATA_EFFPARTENZA"

....