On Cell change, execute macro

Hi,

I'm working on a spreadsheet that will minimise the effort for users that update it. For example they are required to enter a value in cells F7:F560 and on this I want to have my macro update the cell Offset(0, 4) with today's date. see below for example of my code. The problem I'm having is that this seems to run where ever i click in the worksheet and takes about 5 - 6 seconds to run.

Any ideas on how I could make this a bit more efficient to run.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim Thing As Range
Set act = Range("F7:F535")
For Each Thing In act
If Thing.Value = "" Then

Thing.Offset(0, 4) = ""

ElseIf Thing.Value <> "" And Thing.Offset(0, 4) = "" Then

Thing.Offset(0, 4) = Now

End If
Next

End Sub

so if i just put

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before and after the piece that will be fine?