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Seems very similar to a 'watchdog', just... reporting instead of doing anything about it.

There’s no crontab for banking days. Deadcheck also requires checkins within a delta of the expected checking time.

Deadcheck also doesn’t rely on your infra to alert.

Yeah I thought "dead man's switch" was such a weird name for this.

It's something that happens if you fail to take action to prevent it at some specified frequency. Looks a lot like a dead man switch to me.

uptime monitor?

Uptime monitors ping your server, this seems to do the opposite.