portable way to store milliseconds with chrono

I've a code as follows -

int main(){
    ....

    auto time = std::chrono::system_clock::now().time_since_epoch() / std::chrono::milliseconds(1);

    ....
    return 0;
}

The variable time here gives output as l with typeid().name() method, but is it safe to assume that if I replace auto with long type, the variable will still store the correct amount of milliseconds across different machines?

I need it because I cannot specify auto as type in class members, since they aren't constexpr or static where it might've been possible. And my intent is to send the data to browser where I can do var d = new Date(time) and it displays correct time. The communication part has been figured out via json format, I'm only stuck at how to store it correctly across different systems.