You can code this directly in PPT using the same DateDiff and putting the result in a textbox.
Example code to play with
Sub OnSlideShowPageChange(SW As SlideShowWindow)
Dim daysCount As Long
Dim startDate As Date
startDate = CDate("19 June 2018")
' Note to make this reliable add a command button to the first slide
' and set it to not visible in the selection pane
' obviously change the shape name for text
If SW.View.CurrentShowPosition = 1 Then
daysCount = DateDiff("d", startDate, Now)
ActivePresentation.Slides(1).Shapes("My TextBox").TextFrame.TextRange = "Days Accident Free: " & CStr(daysCount)
End If
End Sub