Application.OnTime TimeValue Running at unspecified Times

I want to use an external device to scan and measure voltage. I want to run a scan every 6 hours. The code I used to do that is as follows.

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Private Sub Workbook_Open()
'Runs a voltage scan at 9:00am, 3:00pm, 9:00pm, and 3:00am

Application.OnTime TimeValue("03:00:00"), "Scan34970A"
Application.OnTime TimeValue("09:00:00"), "Scan34970A"
Application.OnTime TimeValue("15:00:00"), "Scan34970A"
Application.OnTime TimeValue("21:00:00"), "Scan34970A"

End Sub

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However, when I run the macro I am having the "Scan34970A" fire at times that are undesired. Also the scan will occasionally run several times when the scan is called at the times specified.

How would I get the scan to ALWAYS run only once, and only at the times requested?

Thank you for the support ,

JD

Instead of having the workbook check what time it is, i recommend using windows task schedule and schedule a task to open the workbook at your designated time. Then change your workbook to run the macro once when it opens.