An RTC keeps your clock counting the cycles of an oscillator, usually an external 32 768 kHz crystal oscillator circuit, an internal oscillator based on a capacitor, or even an embedded quartz crystal. Some can detect transitions and count the periodicity of an input that may be connected. RTC modules are simply date and time memory systems that have a battery configuration that, in the absence of external power, keeps the module running. This keeps the TIME and DATE up to date.
Therefore, we can have the precise TIME and DATE of the RTC module whenever we want.
