[v5] Fix strptime era handling, add more strftime tests [BZ #24394]

[Florian Weimer]

* DJ Delorie:

> +  snprintf (buffer, TMBUFLEN, "%04d/%02d/%02d %02d:%02d:%02d %s",
> +	    tm->tm_year,
> +	    tm->tm_mon,
> +	    tm->tm_mday,
> +	    tm->tm_hour,
> +	    tm->tm_min,
> +	    tm->tm_sec,
> +	    wn);

What's the reason to use this non-standard date format?  Why not ISO
8601 with 0-based years and 1-based months?