c++ - what is the counter part of boost::date_time::not_a_date_time in STL -

note: saw question: what c++11 equivalent boost::date_time::not_a_date_time?. there no rectification. want more specific problem answer.

i have boost::system_time, cleared assigning boost::date_time::not_a_date_time.

class data {     boost::system_time datetime;     void clear(void)     {         datetime = boost::date_time::not_a_date_time;     }     const boost::system_time &getdatetime() const     {         return datetime;     } }; void main() {     data obj;     // processing of data obj     if ( obj.getdatetime() >= )     {         // inform other module     } } 

now, there in stl, can assigned value (let's now()) , cleared invalid value?

tried creating time_point empty constructor. pointing epoch time. see in odd scenarios, time_point have null value. there way achieve (if safe)