cとc++の時間

//clock()                ；
//   ctime         —CLOCKS_PER_SEC ,                  ；
//       CLOCKS_PER_SEC      ；     CLOCKS_PER_SEC               ；
//ctime clock_t  clock()       ，           clock_t;
//      clock()/CLOCKS_PER_SEC              ;
//    clock                               ;
 
//    
#include <iostream>
#include <ctime>
int main()
{
 using namespace std;
 float  secs;
 cin >>secs;
 clock_t delay =secs* CLOCKS_PER_SEC;
 clock_t start =clock();
 while (clock()-start<delay)
 ;
 cout <<"done!
" ;
 system("pause");
  
}

//                             
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
long i = 10000000L;
clock_t start, finish;
double duration;
/*            */
printf( "Time to do %ld empty loops is ", i) ;
start = clock();
while( i-- );
finish = clock();
duration = (double)(finish - start) / CLOCKS_PER_SEC;
printf( "%f seconds
", duration );
system("pause");
}

// ctime    
//    :             
 #include <stdio.h>
#include <time.h>
int main(void)
{
time_t t;
t=time(&t);
printf("Today's date and time: %s
", ctime(&t)); //       
system("pause");
return 0;
}

//    1970 1 1          ；
#include <stdio.h>
#include <time.h>
int main()
{
 time_t t;
 time(&t);
 printf("%d",t);  
 system("pause");
}