Different format of __DATE__ macro

Question

C has a predefined macro __DATE__, that shows the date of the compiled source file .
The date is displayed in the format "Mmm dd yyyy" .

Is there any way to be formatted this date, using macros ?
In this format "yyyy Mmm dd".

Instead of being :

Jul 19 2013

Should be :

2013 Jul 19

Solution 1

In C you could have a macro that generates a compound literal on the fly that has the order that you like, something like

#define FDATE (char const[]){ __DATE__[7], __DATE__[8], ..., ' ', ... , '\0' }

in all places where it matters your optimizer should be able to handle this efficiently.

Solution 2

Here's a true hack:

union {
    const char DOUBLE_DATE[18];
    const char PAD[19];
} DATE_HELPER = { __DATE__ " " __DATE__ };

const char *MY_DATE = DATE_HELPER.DOUBLE_DATE + 7;

Solution 3

I wanted the date of compilation in ISO format in a C++ program, so I came up with this:

constexpr unsigned int compileYear = (__DATE__[7] - '0') * 1000 + (__DATE__[8] - '0') * 100 + (__DATE__[9] - '0') * 10 + (__DATE__[10] - '0');
constexpr unsigned int compileMonth = (__DATE__[0] == 'J') ? ((__DATE__[1] == 'a') ? 1 : ((__DATE__[2] == 'n') ? 6 : 7))    // Jan, Jun or Jul
                                    : (__DATE__[0] == 'F') ? 2                                                              // Feb
                                    : (__DATE__[0] == 'M') ? ((__DATE__[2] == 'r') ? 3 : 5)                                 // Mar or May
                                    : (__DATE__[0] == 'A') ? ((__DATE__[2] == 'p') ? 4 : 8)                                 // Apr or Aug
                                    : (__DATE__[0] == 'S') ? 9                                                              // Sep
                                    : (__DATE__[0] == 'O') ? 10                                                             // Oct
                                    : (__DATE__[0] == 'N') ? 11                                                             // Nov
                                    : (__DATE__[0] == 'D') ? 12                                                             // Dec
                                    : 0;
constexpr unsigned int compileDay = (__DATE__[4] == ' ') ? (__DATE__[5] - '0') : (__DATE__[4] - '0') * 10 + (__DATE__[5] - '0');

constexpr char IsoDate[] =
{   compileYear/1000 + '0', (compileYear % 1000)/100 + '0', (compileYear % 100)/10 + '0', compileYear % 10 + '0',
    '-',  compileMonth/10 + '0', compileMonth%10 + '0',
    '-',  compileDay/10 + '0', compileDay%10 + '0',
    0
};

// Test that it gets it right today
#include <cstring>
static_assert(strcmp(IsoDate, "2020-11-06") == 0);

Solution 4

In case anyone is looking for a way to convert Mmm DD YYYY into a string YYMMDD with ASCII digits only (like "220103"), then here's the code for that. It should work in C and C++ both:

#include <stdio.h>

const char version[6+1] =
{
   // YY year
   __DATE__[9], __DATE__[10],

   // First month letter, Oct Nov Dec = '1' otherwise '0'
   (__DATE__[0] == 'O' || __DATE__[0] == 'N' || __DATE__[0] == 'D') ? '1' : '0',
   
   // Second month letter
   (__DATE__[0] == 'J') ? ( (__DATE__[1] == 'a') ? '1' :       // Jan, Jun or Jul
                            ((__DATE__[2] == 'n') ? '6' : '7') ) :
   (__DATE__[0] == 'F') ? '2' :                                // Feb 
   (__DATE__[0] == 'M') ? (__DATE__[2] == 'r') ? '3' : '5' :   // Mar or May
   (__DATE__[0] == 'A') ? (__DATE__[1] == 'p') ? '4' : '8' :   // Apr or Aug
   (__DATE__[0] == 'S') ? '9' :                                // Sep
   (__DATE__[0] == 'O') ? '0' :                                // Oct
   (__DATE__[0] == 'N') ? '1' :                                // Nov
   (__DATE__[0] == 'D') ? '2' :                                // Dec
   0,

   // First day letter, replace space with digit
   __DATE__[4]==' ' ? '0' : __DATE__[4],

   // Second day letter
   __DATE__[5],

  '\0'
};

int main(void)
{
  puts(__DATE__);
  puts(version);
}

Output:

Jan  3 2022
220103

Disassembly (gcc x86_64):

version:
        .string "220103"

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