Option Explicit
Private Sub Worksheet_Activate()
Dim rngMyUsedRange As Range
Dim rCell As Range
Set rngMyUsedRange = Range("A1:N40")
For Each rCell In rngMyUsedRange
If IsDate(rCell.Value) _
And rCell.Value < Date + 30 Then
rCell.Font.Color = &HFF&
End If
Next
End Sub
...so right-click the sheet's tab, select View Code, and paste in VBIDE.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngMyUsedRange As Range
Dim rCell As Range
Set rngMyUsedRange = Range("A1:N40")
For Each rCell In rngMyUsedRange
If IsDate(rCell.Value) _
And rCell.Value < Date + 30 Then
rCell.Font.Color = &HFF&
End If
Next
End Sub