Stuck with having a vba only apply to a specific slide

[vbacreator]

Sorry a complete noob to vba but I'm using a vba to create a countdown timer, here it is:

Sub countdown()

Dim future As Date

future = DateAdd("n", 30, Now())

Do Until future < Now()

DoEvents

ActivePresentation.Slides(21).Shapes("square").TextFrame.TextRange = Format(future - Now(), "nn:ss")

Loop

End Sub

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Question:

This creates a countdown timer in the shape named "square" on slide 21 (in bold for ease). The problem is sometimes I change the order of the slides so have to respecify the slide each time, is there a way to have this run without needing to specify the slide? I've tried googling this and apparently the trick is to remove the 21 but this doesn't seem to work. Anyone know how to do this? Thanks in advance.