I keep time with a 3.3v coin battery on board battery connector. Set time for software and then sync clock to hardware. For safety and convenience, I keep it on a holder plugged in a very small blue breadboard and blue/white male-female jumper wires... Adds to the look. PS: I don't really exactly know know about the current draw when asleep, but since the end of December, it keeps the time .
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Most coin cells have a capacity somewhere in the range of 250 mA-Hours. The maximum drain of the Quark by RTC is 6 uA. 0.25/0.000006 is 4.7 years. The coin cell at room temperature should last at least that long - probably much longer.
I put a 0.22 F super-cap with a resistor and diode from 3.3 on mine. It should last about 12 hours and charge back up in a couple. It covers power outages and moving the board.