7date.c Print out today's date in the form: <day-of-year base 7 (0-indexed)>.<year>

7date.c

// 7date.c
//
// Print out today's date in the form
// <day-of-year base 7 (0-indexed)>.<year>
//
// Examples:
//
//  Jan 1, 1925  ==  0.1925
//
//  Feb 1, 2025  ==  43.2025
//

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

#define BUF_SIZE 64

// Returns a base 7 string representation of the given number.
// The returned value should be copied unless used before the
// next call to base_7_str.
char *const base_7_str(int n) {
  if (n < 0) return "";
  if (n == 0) return "0";
  static char buf[BUF_SIZE];
  char *c = buf + (BUF_SIZE - 1);
  *c-- = '\0';
  while (n > 0 && c >= buf) {
    *c-- = '0' + n % 7;
    n /= 7;
  }
  if (c < buf) return "";
  return ++c;
}

int main(int argc, char **argv) {
  time_t time_sec;
  time(&time_sec);
  struct tm *t = localtime(&time_sec);
  printf("%s.%d\n", base_7_str(t->tm_yday), t->tm_year + 1900);
  
  return 0;
}

You can compile and run this from bash with the command:

make 7date ; ./7date

You can install it as an always-available command by running this from the same dir that you ran make in:

sudo ln -s `pwd`/7date /usr/local/bin/7date

The above commands assume you have some (usually installed) things like make and a C compiler, and that /usr/local/bin is in your $PATH.

I gave this small program some little upgrades and its own repo:

https://github.com/tylerneylon/7date_C