You'd be better off using a local variable to do the addition and immediate checking on. So, like:
double fiveHundredMillisecondCounter_temp = this->fiveHundredMillisecondCounter + this->deltaFrameTime;
if (fiveHundredMillisecondCounter_temp >= 0.5) {
    // blah blah
}
this->fiveHundredMillisecondCounter = fiveHundredMillisecondCounter_temp;

Same thing below as well.