date.c

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

/*
 * Format characters:
 * %y: 2-digit year
 * %Y: 4-digit year
 * %M: Month name
 * %N: Month number
 * %D: Day of month
 * %d: Day of week name
 * %H: Hour in 24-hour mode
 * %h: Hour in 12-hour mode
 * %m: Minute
 * %s: Second
 * %P: AM or PM label
 * %Z: String representing time zone
 * %n: Newline
 * %%: % symbol
 */

char *format_string = "%d %M %D, %Y %H:%m:%s %Z";
char *months[] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
char *dow[] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

void usage( void );

int main( int argc, char **argv ){
    time_t timestamp = time( NULL );
    struct tm *curtime = localtime( &timestamp );
    if( argv[1] ){
        if( argv[1][0] == '-' ){
            usage();
            exit( 0 );
        }
        format_string = argv[1];
    }
    int len = strlen( format_string );
    /* Scan format string: */
    for( int i = 0; i < len; i++ ){
        if( format_string[i] == '%' ){
            i++;
            switch( format_string[i] ){
                case 'Y' : printf( "%d", curtime->tm_year + 1900 ); break;
                case 'y' : printf( "%s%d", (curtime->tm_year % 100 < 10)?"0":"", curtime->tm_year % 100); break;
                case 'N' : printf( "%s%d", (curtime->tm_mon < 9)?"0":"", curtime->tm_mon + 1 ); break;
                case 'M' : printf( "%s", months[curtime->tm_mon] ); break;
                case 'D' : printf( "%s%d", (curtime->tm_mday < 10)?"0":"", curtime->tm_mday ); break;
                case 'd' : printf( "%s", dow[curtime->tm_wday] ); break;
                case 'H' : printf( "%s%d", (curtime->tm_hour < 10)?"0":"", curtime->tm_hour ); break;
                case 'h' : printf( "%2d", (curtime->tm_hour % 12)?(curtime->tm_hour % 12):12 ); break;
                case 'm' : printf( "%s%d", (curtime->tm_min < 10)?"0":"", curtime->tm_min ); break;
                case 's' : printf( "%s%d", (curtime->tm_sec < 10)?"0":"", curtime->tm_sec ); break;
                case 'P' : printf( "%cM", (curtime->tm_hour < 12)?'A':'P' ); break;
                case 'Z' : printf( "%s", curtime->__TM_ZONE ); break;
                case 'n' : putchar( '\n' ); break;
                default  : putchar( format_string[i] );
            }
        }
        else putchar( format_string[i] );
    }
    putchar( '\n' );
    return 0;
}

void usage(){
    putchar( '\n' );
    puts( "Usage: date [format_string]" );
    putchar( '\n' );
    puts( "Metacharacters for format string:" );
    puts( "\t\%y: 2-digit year" );
    puts( "\t\%Y: 4-digit year" );
    puts( "\t\%M: Month name" );
    puts( "\t\%N: Month number" );
    puts( "\t\%D: Day of month" );
    puts( "\t\%d: Day of week name" );
    puts( "\t\%H: Hour in 24-hour mode" );
    puts( "\t\%h: Hour in 12-hour mode" );
    puts( "\t\%m: Minute" );
    puts( "\t\%s: Second" );
    puts( "\t\%P: AM or PM label" );
    puts( "\t\%Z: String representing time zone" );
    puts( "\t\%n: Newline" );
    puts( "\t\%\%: \% symbol" );
    putchar( '\n' );
    puts( "Default format string:" );
    printf( "\"%s\"\n\n", format_string );
}