Simple Program Giving Negative Result For Amount Of Calculations Done

Why this extremely simple program seems to be giving me a negative value for amount of calculations done within one minute ( Just using it as a bit of fun to see how different computers in the office perform).

Java Code:

class Benchmark {
public static void main(String[] args) {
long endTime = System.currentTimeMillis() + 60000;
int count = 0;
for (int i = 0; System.currentTimeMillis() < endTime; i++) {
double x = Math.sqrt(System.currentTimeMillis());
count = i;
}
System.out.print(count + " calculations per minute");
}
} mh_sh_highlight_all('java');

I am getting results between -2.1billion and -3.4billion which to me would make sense since they are not the best computers in the world but I would be expecting a positive value?