Printing out relative date from current time

Question

Here is some code of mine that prints out the relative date according to the current time:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main (int argc, char *argv[])
{
  int i = 1;
  struct tm date = {0};
  char relativeDays[80];

  char* temp = strtok(argv[1], "/");
  while(temp != 0)
  {
    switch(i++)
    {
                case 1:
                        date.tm_mon  = atoi(temp) - 1;
                        break;
                case 2:
                        date.tm_mday = atoi(temp);
                        break;
                case 3:
                        date.tm_year = atoi(temp) - 1900;
    }
    temp=strtok(NULL, "/");
  }
  i = (int) difftime(time(NULL), mktime(&date))/86400;
  sprintf(relativeDays, "%d", abs(i));
  if (i > 0) printf("%s\n", strcat(relativeDays, " days ago."));
  else if (i < 0) printf("%s\n", strcat(relativeDays, " days from now."));
  else printf("Today\n");
  return 0;
}

---

Sample input and output:

$ ./date 12/8/2013
2 days ago.
$ ./date 12/10/2013
Today
$ ./date 12/24/2013
13 days from now.

Any thoughts on how to improve the code, specifically how to make it shorter?

Answer 1

Date-time handling is tricky to implement correctly. Unless you want to reinvent the wheel, use strptime(3) to parse dates. Your code will be simpler, and you should automatically get validation to reject dates like "12/32/2013".

When converting the input date to a Unix epoch value using mktime(&date), you always interpret the date as midnight in the UTC time zone. That leads to two bugs in calculating the date difference, in my opinion:

1. The user would expect the input date to be interpreted in the local time zone.
2. When specifying tomorrow's date (and interpreting it in the local time zone), the output will be "Today", since 00:00:00 of the following day is less than 86400 seconds in the future.

The output routines could also use some slight improvement. You need neither abs() nor strcat(). Also, it would be human-friendly to handle singular numbers.

The magic number 86400 should be explained better.

#include <stdio.h>
#include <time.h>

#define SECONDS_PER_DAY (24 * 60 * 60L)

int main(int argc, char *argv[]) {
    char *input_str;
    time_t now;
    struct tm input_tm = { 0 };
    long diff_days;

    if (argc <= 1) {
        fprintf(stderr, "Need MM/DD/YYYY input\n");
        return 1;
    }
    input_str = argv[1];

    /* When calling difftime(), both arguments should have the same
       hr:min:sec, else you might get off-by-one-day errors depending on
       the time of day when you run the program.  We could either use
       00:00:00 midnight for both, or the current hr:min:sec for both.
       Let's use the current time for both. */
    time(&now);
    localtime_r(&now, &input_tm);

    /* strptime() will fill in MM/DD/YYYY, but leave the hr:min:sec and
       time zone fields from above alone. */
    if (!strptime(input_str, "%m/%d/%Y", &input_tm)) {
        fprintf(stderr, "Bad date: %s\n", input_str);
        return 1;
    }
    diff_days = (long)difftime(now, mktime(&input_tm)) / SECONDS_PER_DAY;

    if (diff_days > 0) {
        printf("%ld day%s ago.\n", diff_days, (diff_days > 1 ? "s" : ""));
    } else if (diff_days < 0) {
        printf("%ld day%s from now.\n", -diff_days, (diff_days < -1 ? "s" : ""));
    } else {
        printf("Today\n");
    }
    return 0;
}

Answer 2

I've commented your code:

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

int main(int argc, char *argv[])
{
    int i = 1;
    struct tm date = {0};
    char relativeDays[80];

Check For Proper Input

    if(argc != 2)
    {
        printf("Two arguments required!\n");
        return -1;
    }

    char *temp = strtok(argv[1], "/");

temp will be NULL if empty. Also, add a break after case 3: for consistency. If you think you'll need to handle unexpected cases, use default at the end.

    while(temp != NULL)
    {
        switch(i++)
        {
            case 1:
                date.tm_mon  = atoi(temp) - 1;
                break;
            case 2:
                date.tm_mday = atoi(temp);
                break;
            case 3:
                date.tm_year = atoi(temp) - 1900;
                break;
            default:
        }

Be consistent with spacing.

        temp = strtok(NULL, "/");
    }

Add comments so people know what your intentions are. Why are you dividing by 86400?

    i = (int)difftime(time(NULL), mktime(&date)) / 86400;
    sprintf(relativeDays, "%d", abs(i));

Use spaces to make everything more readable.

    if(i > 0)
        printf("%s\n", strcat(relativeDays, " days ago."));
    else if(i < 0)
        printf("%s\n", strcat(relativeDays, " days from now."));
    else
        printf("Today\n");

    return 0;
}

Edit:

This while block can be changed to the following. Though it doesn't change the operation, it may improve the aesthetics.

while(temp != NULL)
{
    int num = atoi(temp);    
    switch(i++)
    {
        case 1:
            date.tm_mon = num - 1;
            break;
        case 2:
            date.tm_mday = num;
            break;
        case 3:
            date.tm_year = num - 1900;
            break;
        default:
            printf("Unexpected input. Blowing up now!\n");
            break;
    }

    temp = strtok(NULL, "/");
}

Answer 3

Concerns if argv[1] represents a local time

1. Should add date.tm_isdst = -1;. Although the h:m:s are set to 0, can't think of a dst change that crossed a day, so this is pedantic.

Other concerns.

1. Should check range of year, easy for folks to enter only last 2 digits and expect 13 to be 2013.