What is the function of pushbutton reset RST pin of DS3231SN?

RST pin has two functions: 1. When VCC falls below VPF, RST is driven low. This is for monitoring the power supply. 2. When connected to an external pushbutton, it can provide debounce function. Assertion of the RST output, whether by pushbutton or power-fail detection, does not affect the internal operation of the DS3231.