An external crystal cannot be used with the DS3231 as the DS3231 was not designed with any designated pins for an external crystal.. However, to improve on the DS3231's accuracy and drift specifications the designer can choose their own RTC source and periodically write that data to the DS3231's clock registers to update them. This can be useful in applications where the designer would like to sync multiple RTC's to one clock source or would just like to improve timekeeping accuracy.