When either button is pressed the PNP transistor turns on and it connects raw battery positive on orange to red. <br>This powers the DC-DC converter which then provides regulated 5V to the "Raw" of the Arduino via the brown wire.<br>The Arduino's internal regulator then powers up the rest of the circuits at 3.3V via "VCC". <p>Programming pulls D2 high to maintain power via the NPN transistor until inactivity timeout.<P>Battery voltage is applied to A1 for display on a page ,which is not shown, by pressing both buttons simultaneously<br>Additional diodes to D5 and D6 monitor push-button presses during operation.

Two of the diodes allow for optional powering via USB to save the batteries. The additional voltage drop does not impair the operation of the Arduino's internal regulator.<p>The third diode prevents excessive voltage from reaching the Arduino. Rx and RST need no protection since it is built into the Arduino.

This RTC is the high-precision version. It shares the I2C bus. While it is  supported by Arduino libraries, mostly only for the low-precision DS1307, I chose not to use any library for brevity. <p>The display also interfaces by I2C and will work at 3.3V.