i would have thought that it is working in Unix time anyway and it would be easy for it just to give you that value anyway...??
But it seems that it is not so easy (well at least needs a bit of code) to get the Unix time out of the DS3231 (or am i missing something here)
DateTime now = rtc.now();uint32_t timestamp = now.unixtime());
Sorry for not saying which library i am using, should have thought about that, i am using the <DS3231.h> library is there the same fetcher for this??