Saving Milliseconds to array

I am running an infinte while loop. Inside the while loop i give a FOR loop which runs 3 times. each time i get a specific value of second from the real time clock. I need to store these 3 seconds value in an array and calculate the average at the end of 3 rd loop.

I tried like this:

int time_diff[3];// Initialization of array for 3 values
double avg;

SCU_APBPeriphClockConfig(__RTC,ENABLE); /*enable RTC clock*/
SCU_APBPeriphReset(__RTC,DISABLE);      /*RTC out of RESET state*/

time.milliseconds = 0; // Initialization time sturcture

while(1)
{
 for(k=0;k<=2;k++)
  {
    if ( a condition)
    {
     RTC_GetTime(BINARY,&BEGIN_TIME);
    }
    if ( another conditon)
    {
    RTC_GetTime(BINARY,&END_TIME);
    time_diff[k] = %02dms; // THIS LINEDIDNT WORK. DONT KNOW how to save that current value to time_diff[k] array
    }
    if (k == 3)
    {
    avg = (time_diff[0]+ time_diff[1]+time_diff[2])/3;
    sprintf(text, "Average =  %d",avg);
    lcd_print (text);
    }
 }
}

How is it possible to save the millisecond value as an array element? This code below displays the millisecond. But what i need is to save this current value to an array and then do averaging.

sprintf(text, "TIME in milli: %02dms ",END_TIME.milliseconds);
lcd_print (text);