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Or just reporpose an old phone.

I was thinking about this in terms of building a product.

Even with that, a clock generally runs for months, even years, on a single battery. Not exactly phone territory, but might be feasible on the ESP8266, depending on how the oscillator behaves in deep sleep mode.

>how the oscillator behaves in deep sleep mode.

horribly

There are many dedicated real time clock chips, pre-designed for backup time-keeping and low power usage. Interfacing with one is going to be way faster/easier/better than trying to roll your own on a general purpose microcontroller, IMO.

TI's bq32000 is $0.55@1K, is 3.3V for easy interface to the ESP, and takes just over 1 microamp in backup power mode. That's just one that I quickly poked at.

(Side note: at this point, I'd think I'd want to be using an ESP32 in any new designs.)

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