What’s the date in ‘n’ days time?

I needed to see if a certificate is due to expire within “n” days. How do I find this date? It turns out to be pretty easy using standard C functions.

                                                                          
#include <stdio.h> 
#include <time.h> 
int main( int argc, char *argv??(??)) 
{ 
....
    char expireDate[11]; 
    time_t t1, t3; 
    struct tm *t2; 
    t1 = time(NULL); 
    t2 = localtime(&t1); 
    t2 -> tm_mday += 40 ; // 40 days from now 
    t3 = mktime(t2); 
    int s; 
    s=  strftime( expireDate, 11, "%Y/%m/%d", t2  ); 
    printf("====size  %d================\n",s); 
    printf(".%10.10s\n",expireDate); 

This successfully printed out the date 40 days, from now. The only little problem I had, was with strftime. The size of the output is 10 bytes. The “11” specifies the maximum number of characters that can be copied into the array. If this was 10… the size of the data I was expecting, The output was wrong “. 2023/06/1” ; off by one character in the buffer and a leading blank.!

With the technique of changing the value within a tm structure you can get the date-time n seconds / m minutes / d days/ from now either in the future – or in the past.

Clever stuff !