It seems like strftime() unexpectedly modifies errno, which is always set to EINVAL when it returns. Looks like it's not related to any specific format.

Here's an example:

#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <errno.h>

int main(int argc, char *argv[])
{
time_t now = time(NULL);
struct tm *tm = localtime(&now);
char buf[100];

errno = 0;
size_t len = strftime(buf, sizeof(buf), "%d %b %Y %H:%M:%S.", tm);

printf("len=%zu\n", len);
printf("buf=%s\n", buf);
printf("errno=%d\n", errno);
}