easy way to get pretty times in c++

prettyTime.cpp

const char * niceTime(time_t targetTime, time_t now = time(0)) {
    const char *periods[8] = {"second", "minute", "hour", "day", "week", "month", "year", "decade"};
    float lengths[7] = {60.0, 60.0, 24.0, 7.0, 4.35, 12.0, 10};
    
    int difference = now > targetTime ? now - targetTime : targetTime - now;
    
    int i = 0;
    for (; difference >= lengths[i] && i < 7; i++) {
        difference /= lengths[i];
    }
    
    difference = round(difference);
    
    std::string period = periods[i];
    
    if (difference != 1) {
        period += "s";
    }
    
    std::ostringstream result;
    result << difference << " " << period;
    return result.str().c_str();
}

// usage
niceTime(1404377294); // returns result based on current time
niceTime(1404377294, 1404376994); // returns result based on specific time, in this case "5 minutes"