功能：获取两个时间相差的天数
输入：const string类型的起始时间，const string类型的结束时间 格式为： 2008-09-01
输出：返回两个相差的天数整数

int GetDay(const string& f,const string& s){     //struct tm *time_now;     time_t first,second;     struct tm temp;     int year, month, day;     sscanf(f.c_str(), "%d-%d-%d", &year, &month, &day);     temp.tm_year = year - 1900;     temp.tm_mon = month -1;     temp.tm_mday = day;     temp.tm_hour = 12;     temp.tm_sec = 0;     temp.tm_min = 0;     first = mktime(&temp);          sscanf(s.c_str(), "%d-%d-%d", &year, &month, &day);     temp.tm_year = year - 1900;     temp.tm_mon = month -1;     temp.tm_mday = day;     temp.tm_hour = 12;     temp.tm_sec = 0;     temp.tm_min = 0;     second = mktime(&temp);          return (first-second) / (24 * 60 * 60 );      }