Script runs on other workbooks

I have a function that does some stuff to a bunch of cells every 15 seconds.

EventMacro is called when the workbook in question is opened.
The script does its job great on the workbook its supposed to run on.

But the problem is this: if I have the book with this script open, then hit ctrl+n and open a new workbook, I suddenly get a "subscript out of range error" from my updateDates function. Whats causing this? why does my macro want to run on other sheets?
Thanks for the advice.

Sub EventMacro()
Call updateDates
alertTime = Now + TimeValue("00:00:15")
Application.OnTime alertTime, "EventMacro"
End Sub

Private Sub updateDates()

Dim ws As Worksheet
Set ws = Worksheets("Data")

If Not (ws Is Nothing) Then


For Counter = 10 To 1000
If (IsEmpty(ws.Cells(Counter, 2).Value)) Then 'if no date entered
If Not IsEmpty(ws.Cells(Counter, 4)) Then 'and a value is in the cell
ws.Cells(Counter, 2).Value = Now 'enter today's date
Else
GoTo last 'stops the script from running too far, if both date and data are blank skip out of the loop
End If
End If
Next Counter

End If

last:
End Sub

Hi,

Set ws = Worksheets("Data") works on the 'Active Workbook', which is the workbook you just opened.

If the code you are running is in the file that you want to update, then you probably want:

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'ThisWorkbook' refers to the workbook that contains the code that is running.

Lewis

Thanks!! That did the trick.