POSIX strftime for Perl6

strftime.pm6

use v6;
use NativeCall;
constant MAX_BUF_SIZE = 255; # Seems reasonable
constant time_t = size_t; # Best guess.
my class tm is repr('CStruct') { }; # Fields can be added when needed
my sub gmtime(time_t is rw -->tm) is native { * };
my sub strftime(utf8, size_t, Str, tm -->size_t) is native { * };

sub PosixFormat(DateTime:D $time, Str $format) is export {
    my time_t $t = $time.posix;
    my utf8 $b .= allocate(MAX_BUF_SIZE);
    my $res = strftime($b, $b.bytes, $format, my $tm = gmtime($t));
    $b.subbuf(0,$res).decode;
}
# Test with:
#$ perl6 -I. -e 'use strftime; DateTime.now.&PosixFormat("%a, %d %b %Y %T GMT").say';
#
# augmenting DateTime is left as an exercise to the reader.