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An RTC would have been a more correct solution. This was a "curiosity" project, and counting milliseconds seemed like the simplest approach. I didn't want to have to worry about using libraries, or complications dealing with not enough pins.
The (cheap) way I've always been told is to inflate a balloon inside the wall just enough so that it hold itself there without sticking out of the hole, and then you have something to apply putty to.
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