Using VBA to make a cell blink

How about this:

Dim NextBlink As Double

Sub StartBlink()
    NextBlink = Now + TimeValue("00:00:01")
    Application.OnTime NextBlink, "BlinkCell"
End Sub

Sub BlinkCell()
    Dim ws As Worksheet
    Set ws = ThisWorkbook.Sheets("Sheet1") ' Change "Sheet1" to your sheet name

    Dim cell As Range
    For Each cell In ws.Range("G2:G" & ws.Cells(ws.Rows.Count, "G").End(xlUp).Row)
        If cell.Value <> "" And DateDiff("d", cell.Value, Now) > 45 Then
            With ws.Cells(cell.Row, "D")
                If .Interior.Color = RGB(255, 255, 255) Then
                    .Interior.Color = RGB(255, 0, 0)
                Else
                    .Interior.Color = RGB(255, 255, 255)
                End If
                .Font.Color = RGB(255, 255, 255)
            End With
        End If
    Next cell

    StartBlink
End Sub

Sub StopBlink()
    On Error Resume Next
    Application.OnTime NextBlink, "BlinkCell", , False
End Sub

Also, keep in mind that blinking may be dangerous to users who are susceptible to epileptic seizures.

And it becomes annoying very quickly.

Finally: the existing conditional formatting overrules colors you set using VBA code.

HansVogelaarany ideas on how to make this work?

I know this isn't blinking but unless you give me a compelling reason why a bright red cell isn't obvious enough and you must have blinking I just don't want to encourage that usage. 

This advice from m_tarler is very wise. I second it.

If you search for Excel VBA cell blink I'm sure you can find some code to do it.  It is that hard but I highly discourage it.  Excel has built in Conditional Formatting that would be much more efficient and will be compatible going forward.  VBA is not supported in web Excel or some other platforms and is disabled by default so anyone opening such a sheet has to open it in desktop version and allow the macros to run.

In conditional formatting you can highlight column G and then choose a custom formula and simply type 

=F1+45<G1

and then choose bright red or something very obvious.

I know this isn't blinking but unless you give me a compelling reason why a bright red cell isn't obvious enough and you must have blinking I just don't want to encourage that usage.  Remember if someone opens the sheet and does NOT allow macros then they will not see that blinking but with conditional formatting supported across platforms they will see the bright red cells.