compareTo method
Comparison based on millis.
Implementation
@override
int compareTo(Object that) {
int thisMillis = this.millis();
int thatMillis = (that as HDateTime).millis();
if (thisMillis < thatMillis) {
return -1;
} else if (thisMillis > thatMillis) {
return 1;
}
return 0;
}