You are lucky man. 
I got many DS3231 clones, at least 3 out of 20 and a monthly difference of more than 1 minute.
Of course, I try calibration it, but it couldn't. These are surprisingly missing some registers!
Write commands are ignored and read commands always return the same value.
Certainly some have excellent accuracy, but they loses accuracy in high temperatures condition such as inside a car.
Not surprisingly, there are probably many clone variants in China.