Is there a low voltage LED alternative?
E-paper

E-paper is as low power a technology as possible. because it does not need power to maintain its display. If you do not display the seconds on your clock, you could have your microcontroller sleep for a vast majority of the time, extending battery life drastically.

you could also add the LEDs for night-time display, but have them run from a second battery to avoid compromising the main battery's life.