use POSIX qw(strftime);
use Text::Wrap;
%fmt = (
    ''   => '',
    '%%' => q"A literal `%' character.",
    '%+' => 'The date and time in date(1) format. (TZ)',
    '%a' => 'The abbreviated weekday name according to the current loc
+ale.',
    '%A' => 'The full weekday name according to the current locale.',
    '%b' => 'The abbreviated month name according to the current local
+e.',
    '%B' => 'The full monthname according to the current locale.',
    '%C' => 'The century number (year/100) as a 2-digit integer. (SU)'
+,
    '%c' => 'The preferred date and time representation for the curren
+t locale.',
    '%d' => 'The day of the month as a decimal number (range 01 to 31)
+.',
    '%D' => 'Equivalent to %m/%d/%y. (Yecch - for Americans only. Amer
+icans should note t'
      . 'hat in other countries %d/%m/%y is rather common. This means 
+that in interna'
      . 'tional context this format is ambiguous and should not be use
+d.) (SU)',
    '%F'  => 'Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
+',
    '%G'  => 'The ISO 8601 year with century as a decimal number. The 
+4-digit year corresp'
      . 'onding to the ISO week number (see %V). This has the same for
+mat and value a'
      . 's %y, except that if the ISO week number belongs to the previ
+ous or next yea'
      . 'r, that year is used instead. (TZ)',
    '%g' => 'Like %G, but without century, i.e., with a 2-digit year (
+00-99). (TZ)',
    '%h' => 'Equivalent to %b. (SU)',
    '%H' => 'The hour as a decimal number using a 24-hour clock (range
+ 00 to 23).',
    '%I' => 'The hour as a decimal number using a 12-hour clock (range
+ 01 to 12).',
    '%j' => 'The day of the year as a decimal number (range 001 to 366
+).',
    '%k' => 'The hour (24-hour clock) as a decimal number (range 0 to 
+23); single digits '
      . 'are preceded by a blank. (See also %H.) (TZ)',
    '%l' => 'The hour (12-hour clock) as a decimal number (range 1 to 
+12); single digits '
      . 'are preceded by a blank. (See also %I.) (TZ)',
    '%m'  => 'The month as a decimal number (range 01 to 12).',
    '%M'  => 'The minute as a decimal number (range 00 to 59).',
    '%n'  => 'A newline character. (SU)',
    '%p'  => q"Either `AM' or `PM' according to the given time value, 
+or the corresponding "
      . q"strings for the current locale. Noon is treated as `pm'and m
+idnight as `am'.",
    '%P' => q"Like %p but in lowercase: `am' or `pm' or a correspondin
+g string for the cur" . q"rent locale. (GNU)",
    '%r' => q"The time in a.m. or p.m. notation. In the POSIX locale t
+his is equivalent to" . q" `%I:%M:%S %p'. (SU)",
    '%R' => 'The time in 24-hour notation (%H:%M). (SU) For a version 
+including the secon' . 'ds, see %T below.',
    '%S' => 'The second as a decimal number (range 00 to 61).',
    '%s' => 'The number of seconds since the Epoch, i.e., since 1970-0
+1-01 00:00:00 UTC. ' . '(TZ)',
    '%T' => 'The time in 24-hour notation (%H:%M:%S). (SU)',
    '%t' => 'A tab character. (SU)',
    '%u' => 'The dayof the week as a decimal, range 1 to 7, Monday bei
+ng 1. See also %w. ' . '(SU)',
    '%U' => 'The week number of the current year as a decimal number, 
+range 00 to 53, sta'
      . 'rting with the first Sunday as the first day of week 01. See 
+also %V and %W.',
    '%V' => 'The ISO8601:1988 week number of the current year as a dec
+imal number, range '
      . '01 to 53, where week1 is the first week that has at least 4 d
+ays in the curr'
      . 'ent year, and with Monday as the first day of the week. See a
+lso %U and %W. ' . '(SU)',
    '%w' => 'The dayof the week as a decimal, range 0 to 6, Sunday bei
+ng 0. See also %u.',
    '%W' => 'The week number of the current year as a decimal number, 
+range 00 to 53, sta'
      . 'rting with the first Monday as the first day of week 01.',
    '%X' => 'The preferred time representation for the current locale 
+without the date.',
    '%x' => 'The preferred date representation for the current locale 
+without the time.',
    '%Y' => 'The year as a decimal number including the century.',
    '%y' => 'The year as a decimal number without a century (range 00 
+to 99).',
    '%Z' => 'The time zone or name or abbreviation.',
    '%z' => 'The time-zone as hour offset from GMT. Required to emit R
+FC822-conformant da'
      . 'tes (using "%a, %d %b %Y %H:%M:%S %z"). (GNU)',
);
print "\nGood on $^O\n----------------\n", @bad;
foreach my $fmt ( sort { lc $a cmp lc $b } keys %fmt ) {
    my $result = strftime( $fmt, localtime $^T );
     (my $ofmt=$fmt)=~s/^%(.+)$/\%\#$1/;
    my $result2 = strftime( $ofmt, localtime $^T );
    my $str = join '', "$fmt '$result' ($ofmt '$result2')\n", wrap( "\
+t", "\t", $fmt{$fmt} ), "\n";
    if ($result) {
        print $str;
    } else {
        push @bad, $fmt;
    }
}
print "\nBad on $^O\n----------------\n@bad\n";
[download]

Good on MSWin32
----------------
%% '%' (%#% '%')
    A literal `%' character.
%a 'Sat' (%#a 'Sat')
    The abbreviated weekday name according to the current locale.
%A 'Saturday' (%#A 'Saturday')
    The full weekday name according to the current locale.
%B 'February' (%#B 'February')
    The full monthname according to the current locale.
%b 'Feb' (%#b 'Feb')
    The abbreviated month name according to the current locale.
%c '02/22/2003 12:03:45 AM' (%#c 'Saturday, February 22, 2003 12:03:45
+ AM')
    The preferred date and time representation for the current locale.
%d '22' (%#d '22')
    The day of the month as a decimal number (range 01 to 31).
%H '00' (%#H '0')
    The hour as a decimal number using a 24-hour clock (range 00 to
    23).
%I '12' (%#I '12')
    The hour as a decimal number using a 12-hour clock (range 01 to
    12).
%j '053' (%#j '53')
    The day of the year as a decimal number (range 001 to 366).
%m '02' (%#m '2')
    The month as a decimal number (range 01 to 12).
%M '03' (%#M '3')
    The minute as a decimal number (range 00 to 59).
%p 'AM' (%#p 'AM')
    Either `AM' or `PM' according to the given time value, or the
    corresponding strings for the current locale. Noon is treated as
    `pm'and midnight as `am'.
%S '45' (%#S '45')
    The second as a decimal number (range 00 to 61).
%U '07' (%#U '7')
    The week number of the current year as a decimal number, range 00
    to 53, starting with the first Sunday as the first day of week 01.
    See also %V and %W.
%W '07' (%#W '7')
    The week number of the current year as a decimal number, range 00
    to 53, starting with the first Monday as the first day of week 01.
%w '6' (%#w '6')
    The dayof the week as a decimal, range 0 to 6, Sunday being 0. See
    also %u.
%x '02/22/2003' (%#x 'Saturday, February 22, 2003')
    The preferred date representation for the current locale without
    the time.
%X '12:03:45 AM' (%#X '12:03:45 AM')
    The preferred time representation for the current locale without
    the date.
%Y '2003' (%#Y '2003')
    The year as a decimal number including the century.
%y '03' (%#y '3')
    The year as a decimal number without a century (range 00 to 99).
%z 'W. Europe Standard Time' (%#z 'W. Europe Standard Time')
    The time-zone as hour offset from GMT. Required to emit
    RFC822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z"). (GNU)
%Z 'W. Europe Standard Time' (%#Z 'W. Europe Standard Time')
    The time zone or name or abbreviation.

Bad on MSWin32
----------------
 %+ %C %D %F %g %G %h %k %l %n %P %r %R %s %t %T %u %V
[download]