For n =  10  To  280 
      If Range("A" & n).Value2 = CDbl(FirstDate) Then WithCell = Range("A" & n).Address
     Next

Sub sad1()
    Dim FirstDate As Date, WithCell As String
    FirstDate = DateAdd("m", - 3 , [A1])
    With ActiveWorkbook.Sheets("Операц").Range("A10:A280")
        If IsError(Application.Match(CDbl(FirstDate), .Cells,  0 )) Then Exit Sub
        WithCell = .Cells(Application.Match(CDbl(FirstDate), .Cells,  0 )).Address
    End With
    Debug.Print WithCell
End Sub

Sub test()
    Dim FirstDate As Date, LastDate As Date, WithCell
    With ActiveWorkbook.Sheets( 1 )
        FirstDate = DateAdd("m", - 3 , .[A1])
        LastDate = DateAdd("m",  6 , .[A1])
        WithCell = .[A10:A280].Find(FirstDate, , xlValues).Address
    End With
    Debug.Print WithCell
End Sub