#!/usr/local/bin/perl
use DateTime::Format::Natural;

my @dates = ('Yesterday', '21st August 2009', '2009-07-17 2:35pm');
my $output_format = '%d/%m/%Y %H:%M:%S';

foreach my $date (@dates) {
    my $parser = DateTime::Format::Natural->new;
    my $dt = $parser->parse_datetime($date);
    print $dt->strftime($output_format), "\n";;
}
[download]

Output

31/08/2009 00:00:00
21/08/2009 00:00:00
17/07/2009 14:35:00
[download]