In designing anything, you must start with specifications. That way you can avoid going a long way down a path only to discover that you "can't get there from here" because of some inappropriate choice you've made along the way.

Now, most microcontrollers of the sort you've described so far require a power supply. The microcontroller has a datasheet, usually easy to obtain from the manufacturer, of, perhaps by following one of the "links" at the left of this screen, that will describe the power requirement.

Some of the available 805x MCU's will operate from a single 3-volt lithium battery which will help you because you then don't have to provide a voltage regulator. That is one of the specifications that appear in the datasheet. Further, the www can help you find out about various sorts of batteries.

The simplest circuit that you can build that will do what you want and do so with any sort of precision, will require that you operate the MCU from a crystal. That will provide a timebase, and, as far as accuracy, it is not difficult if you pay close attention to the specifications and recommendations provided by component manufacturers, to create a crystal oscillator that stays within 100 ppm (parts per million) of the nominal frequency over typical temperature and voltage range. The crystal oscillator is provided, generally speaking in the MCU, with the exception of a couple of capacitors and, sometimes, an external resistor, as well as the crystal.

I'm generalizing, of course, but most 3-volt or 3.3-volt CMOS MCU's will drive a 7-segment LCD of the type I previously mentioned directly from the port pins, therefore demanding no external components. Since there is no DC-path from the segment inputs on the LCD to the backplane, no series resistors are required. You must, however, be aware of what is going on in the LCD.

There's plenty of material on that available on the www, so I won't bother you with that now.

Specifications will tell you how long you can run your chosen MCU on a given battery, as the battery will have a sort-of "useful charge-life" rating usually expressed in mAH (milliampere hours) If you understand the specifications of the battery, and how to interpret them, and the specifications of the MCU, you should have no trouble figuring out how long you can use a battery before it is discharged to beyond its useful output.

Keep in mind that power consumption is a factor of speed. If you reduce speed, you reduce power consumption. This becomes a circular problem, in that you have to know how fast you must run the MCU in order to execute the code fast enough to keep the display functioning. (If you can execute your display loop in 30 milliseconds, you'll be fine, just in round numbers).

Do the reading. There's plenty of it! The worst that can happen is that you learn something.

RE