День программиста

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

int main(int argc, char const **argv)
{   
    char st[16];
    time_t t;
    struct tm tm = {0};
    int y = 1970;
    if (argc > 1 && (y = atoi(argv[1])) < 1900)
    {   
        fprintf(stderr,"Year *MUST* be an integer >=1900: '%s'\n",argv[1]);
        return 1;
    }
    tm.tm_year = y - 1900;
    tm.tm_mday = 1; 
    t = timegm(&tm) + 255*24*3600;
    (void)gmtime_r(&t,&tm);
    strftime(st,sizeof(st)-1,"%Y-%m-%d",&tm);
    printf("%s\n",st);
    return 0;
}