c++ 获取系统时间减去天数

//计算日期节点 nSaveDays为天数
    CTime tmDeleteday=CTime::GetCurrentTime() - CTimeSpan(nSaveDays, 0, 0, 0 );//获取系统当前时间 并 减去相应的天数得到 想要的时间节点
    CString strDeleteday;
    strDeleteday.Format("%04d-%02d-%02d", tmDeleteday.GetYear(), tmDeleteday.GetMonth(), tmDeleteday.GetDay());//格式化为想要的形式

下面是一个根据配置文件删除数据库中ABC表中超过设定天数的数据栗子：

//删除数据库中ABCInfo表中超过设定天数的数据
short CPeristentData::DeleteABCInfo()
{
    LOGINFO("CPeristentData::DeleteABCInfobegin");

    //从配置文件获取天数
    int nSaveDays = GetPrivateProfileInt("DELETESNRINFO","SaveDays",0,INIFILE);//INIFILE为宏定义配置文件路径
    
    if (nSaveDays <= 0)
    {
        nSaveDays = 90;//默认为90天
        WritePrivateProfileString("DELETEABCINFO","SaveDays","90",INIFILE);//没有则写入配置文件，以便于维护
    }

    //计算删除的日期节点
    CTime tmDeleteday=CTime::GetCurrentTime() - CTimeSpan(nSaveDays, 0, 0, 0 );
    CString strDeleteday;
    strDeleteday.Format("%04d-%02d-%02d", tmDeleteday.GetYear(), tmDeleteday.GetMonth(), tmDeleteday.GetDay());

    if (!OpenDatabase())//是否打开了数据库
    {
        return -2;
    }

    char szSql[1024];
    //delete from ABCInfo where OperationTime < '2018-04-01'
    sprintf(szSql,  "delete from ABCInfo \
                    where OperationTime<\'%s\'", 
                    strDeleteday);

    if(!m_Sqlite.ExcuteNonQuery(szSql))//数据库操作
    {
        m_Sqlite.CommitTransaction();
        LOGINFO("ExcuteNonQuery Result: Error---[%s]",szSql);
        return -3;
    }

    LOGINFO("CPeristentData::DeleteABCInfo end");

    return 0;
}