DateAdd Equivalent in C

can you tell me what structure you are using for representation of the date?

Maybe this helps some,

I am reading a file and need to find yesterdays date with a line. So I am trying to use the time function in c to give me yesterdays date to  compare in the file. I have something like this to get the current date into the format the date is in the file.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <time.h>

struct tm *ptr;
time_t lt;

int main(int argc, char** argv) {

    t = time(NULL);

    ptr = localtime(&lt);

    strftime(dt, 75, "%Y/%m/%d", ptr);

   printf(Date: %s \n, dt);

   return (EXIT_SUCCESS);
}

This will give we the date obviously, just not sure how to subtract a day from the calendar to accommodate for month and year.

working with tm is a bit hard, since it is a struct.
I would recomend you to work with time_t which is long. Here the date is represented in seconds passed from (00:00:00), January 1, 1970.
So if you subtrctract 60sec*60min*24h==1day=86400sec from the lt, you will get the previous day

t=t-86400;