[pdq]

These would make a great wristwatch, so you can get perpetual display of time with virtually zero power, and should get long battery life.

[Tharkun]

I believe Nokia/Withings offers a fitness tracker with an eink display with a watchface.

[r3bl]

That is correct. Nokia/Withings Steel HR on my hand contains a small, surprisingly fast e-ink display (although low res).

They also have small, barely noticeable LED lights lighting the screen to be visible during night. Much more pleasant way of checking the time during the night than looking at a phone. And its battery lasts 2-3 weeks and charges in two hours.

The screen isn't always on though, and only activates on a click from the side.