how to input a time which is in the future through time.h .....and how to find time difference between the "check in" and "check out time"?

struct tm * timeinfo;
 
  time ( &rawtime );
  timeinfo = localtime ( &rawtime );
 cout<<"The Check in time is : " <<asctime (timeinfo);
 ofstream customerfile ("customers.txt", ios::out | ios::app );
 
string day;
string month;
string date;
cout<<"Please fill the check out time data :"<<endl;
 
cout<<"The Day of check out : \n";
cin>>day;
cout<<"The Month of check out : \n";
cin>>month;
cout<<"The Date of check out  : \n";
cin>>date;
string chkout = day + month + date + (asctime (timeinfo)+10);

if (customerfile.is_open())
{
customerfile<< asctime (timeinfo)<<","<< chkout<<"\n";}
customerfile.close();
				
				
				
				system ("pause");
				goto Main_Menu;
				break;
			}