unsigned int get_day(unsigned int timestamp)
{
    time_t t = timestamp;
    struct tm * timeval = gmtime(&t);
    return timeval->tm_wday;//0 = sun
}

unsigned int get_start_of_day(unsigned int timestamp)
{
    int day_of_w = get_day(timestamp);
    while(get_day(timestamp-3600) == day_of_w) timestamp -= 3600;
    while(get_day(timestamp-60) == day_of_w) timestamp -= 60;
    while(get_day(timestamp-1) == day_of_w) timestamp--;

    return timestamp;
}