VBA Time Stamp for single cell entry

what would the VBA be to enter a date in a single cell when a template file is opened and keep that date frozen after it is saved?
If I need to set a condition, maybe something like if A1>0,Now() formatted for just the date.

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In the module ThisWorkbook :

Code:

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Option Explicit

Private Sub Workbook_Open()
    If Sheet1.Range("A1").Value > 0 Then
        Sheet1.Range("B1").Value = Now()
    End If
End Sub

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