Update: It looks like the app isn't compatible with some versions of the M8 yet either. Feel free to chime in if any of your Sense 6 devices are currently listed as compatible.
Dear M8 owners, HTC has dropped your default clock app into the Play Store. This will allow for easier updates in the future, untangling software improvements from big firmware releases. It's not a particularly exciting app, but hey, the essentials are important too.