i don't know how to sum up two variables of clock_t

[ameni]

#include<stdio.h>
#include<time.h>
main()
{
clock_t start,end,d;
int i,l1 ;
char t2[10];
while(i<l1&&d<=15)
{
i=i+1;
start=clock();
printf("t2[%d]=\n",i);
scanf(" %c",&t2);
end=clock();
double extime=(double) (end-start)/CLOCKS_PER_SEC;
printf("\n\tExecution time for the bubble sort is %f seconds\n ",extime);
double d=(double)(d+extime)/(double)CLOCKS_PER_SEC;


if(d==15){

c1=c1+1;}
}


printf("\n\tExecution time for the bubble sort is %f seconds\n ",d);
printf("\n");