Lowest power consumption for a constantly changing display like a clock is probably a low-voltage LCD display with static drive (that means one common and many segment wires- one per segment), and of minimal area (obvious trade-off here that you'd like to be big enough to read). Low voltage also limits your ambient temperature range, but static displays are much better than multiplexed ones for temperature change to begin with. Alternatively, the off-the-shelf e-ink displays claim a typical current during the change of state (which might last 0.5-2 seconds) of 500nA/cm^2.
The static LCD displays draw continuous power that is related to the area, the square of the voltage, and the refresh frequency (usually something like 50Hz). Multiplexed LCD displays require a voltage divider to get the different voltage levels so they tend to draw more power at a system level.
You can drive static LCD displays directly with a microcontroller if it has enough pins (you have to generate symmetric waveforms that have no more than bout 50mV of DC component or the display can be damaged by electrochemical action, or some low power micros such as PIC and MSP430 have built-in LCD controllers (just write a bit to the register to turn a segment on).
You can drive e-ink displays at 5V directly from a suitable microcontroller (not a 3.3V type), or to full contrast they need 15V and a high-voltage driver. They also require DC balance on the drive but that simply amounts to pulsing the segments 'off' for as long as pulsing them 'on', and keeping track of the current state so you don't write them on more than once without turning them off first. You will also need n+1 pins to drive an n-segment e-ink display.
Chances are either type of display would work for you and would not represent the largest current draw in your system. Static LCDs are more familiar and would change faster from 5V so I think I'd tend that way for a clock display.