What is causing this warning in Arduino IDE?
The warning was caused by this line:

char *monthShort[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};


In C++, the content of a character string such as "Jan" cannot be
modified. Thus, if you assign the string to a pointer, the pointer
should be declared as const&nbsp;char&nbsp;*:

const char *monthShort[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun",
                            "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};


The same applies to the other arrays of strings: daysLong, daysShort
and monthLong. As a consequence, the prototypes of the methods
getDOWStr() and getMonthStr() should be changed in order to return a
const char *.

BTW, are you sure this library is still maintained? This should have
been fixed a long time ago, and it seems the source was last modified in
2015. You may be better off using a more modern alternative. For
instance, in the Arduino library manager, you can find DS3231 and
Adafruit's RTClib.