POSIX functions are XS wrappers around C runtime library routines of the same name. The difference almost certainly arises because of differences in the different C-runtimes installed on the two platforms. Newer versions of MS CRTs are much stricter about parameter checking.

As for why no error is raised, it appears to be a side-effect of Perl's attempts to be portable. There doesn't appear to be a code path in there for the situations where CRT strftime() returns 0 for reasons other than buffer overflow?

  buflen = 64;
  Newx(buf, buflen, char);
  len = strftime(buf, buflen, fmt, &mytm);
  /*
  ** The following is needed to handle to the situation where
  ** tmpbuf overflows.  Basically we want to allocate a buffer
  ** and try repeatedly.  The reason why it is so complicated
  ** is that getting a return value of 0 from strftime can indicate
  ** one of the following:
  ** 1. buffer overflowed,
  ** 2. illegal conversion specifier, or
  ** 3. the format string specifies nothing to be returned(not
  **      an error).  This could be because format is an empty string
  **    or it specifies %p that yields an empty string in some locale.
  ** If there is a better way to make it portable, go ahead by
  ** all means.
  */
  if ((len > 0 && len < buflen) || (len == 0 && *fmt == '\0'))
    return buf;
  else {
    /* Possibly buf overflowed - try again with a bigger buf */
    const int fmtlen = strlen(fmt);
    int bufsize = fmtlen + buflen;

    Newx(buf, bufsize, char);
    while (buf) {
      buflen = strftime(buf, bufsize, fmt, &mytm);
      if (buflen > 0 && buflen < bufsize)
    break;
      /* heuristic to prevent out-of-memory errors */
      if (bufsize > 100*fmtlen) {
    Safefree(buf);
    buf = NULL;
    break;
      }
      bufsize *= 2;
      Renew(buf, bufsize, char);
    }
    return buf;
  }
#
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In general, the POSIX module provides access to system's implementation (whereas the builtin functions provide portability).

Isn't POSIX supposed to give platform-independent results?

If your platform doesn't conform to POSIX, that's an issue to take up with you platform's provider.

Thanks, but how to get a desired date string was not my question. I was wondering if anybody could help me understand the difference in behavior between two platforms.

I believe japhy's solution suggested that taking the lowercase of the string, made some difference in some systems that either ignore case, or make a big deal of it. Like the difference between old windows and a unix system. I thought I would just point that out. There may be a subtle difference, which you may be overlooking.

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I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh