Hi there,
You could absolutely start a " Xiao Esp32C3 with RTC DS3231 " thread
if you like. Also definitely look over the @grobasoz example for the keypad info on the C3 , it throws everyone as it’s not a lot like the other variants so often code that runs a Vanilla C3 or the Dev board does NOT run on a Xiao or other C3 platforms, without some adjustments.
The Constant rebooting thing YOUR facing is that the D0 is one of the “Strapping Pins” on the ESP32 series. Check out the WiKi for more info on it. But basically (D0) can’t be in a certain state while resetting or the chip will think it’s in Boot mode or an Undetermined state.
In a nutshell you want it to sleep until a certain date or time then wake up ? via the external RTC Int. ? is that correct ?
Keep in mind also the BSP you sue on some examples may require older versions, I like the 2.1.14 seemed to be the one that worked always.
HTH
GL 
PJ 