The RTC (BM8563) is a separate chip on the M5Dial board (it is also used on some other M5Stack products).
It is literally hardwired to the battery which means you can keep track of the clock even while the M5Dial board is shut down - that is as long as you always keep an un-depleted battery connected to the board. The BM8563 chip consumes less than 3 µA@"stdLiIonVoltage"