use Date::Manip;

$diff = &DateCalc("now", "21st at midnight");
$seconds = &Delta_Format($diff, 0, "%st");
[download]

This sounds pretty simple to achieve without modules. I won't write the code, but here's how I'd do it.

Use time to get the current epoch seconds
Use localtime to convert that to date/time
If the date is greater than 21 
then 
  increment the month number
end
set the date to 21 and the time to 12am
use Time::Local::timelocal to convert that time to 
   epoch seconds
Subtract that from your first number
[download]

Originally posted as a Categorized Answer.

You'd have to

Originally posted as a Categorized Answer.

POSIX example

#!/usr/bin/perl 
use strict;
use warnings;
use POSIX (); 


my ( $sec, $min, $hour, $mday, $mon, $year, $wday, $yday, $isdst ) = 
  POSIX::localtime(time);

print POSIX::difftime(
    POSIX::mktime(
        0, 0, 12, 21, 
        ( $mday >= 21 ? ( ( $mon + 1 ) % 11 ) : $mon ),
        ( ( $mon == 11 and $mday >= 21 ) ? $year + 1 : $year )
    ),  
    POSIX::mktime( $sec, $min, $hour, $mday, $mon, $year )
  ),  
  "\n";
[download]

#!/usr/bin/perl

use strict;
use warnings;

use Date::Manip;

my $now = ParseDate 'now';
my $then = ParseDate 'next month';
substr($then,6,2) = 21;
my $delta = DateCalc $now, $then;
my $seconds = Delta_Format $delta, 0, '%sh';
print "seconds=$seconds days=", $seconds/(60*60*24), "\n";
[download]

Directly hacking the Date::Manip date format to stick a 21 in the day field is ugly, but couldn't remember how to do it with the API.

Two examples. (didactical purpose)

#!/usr/bin/perl 
use strict;
use warnings;
use v5.10.0;
use Time::Piece;
use Time::Seconds;
use DateTime;


my $dt = DateTime->now( time_zone => 'floating' );

my $next = { 
    mon => ( $dt->mday >= 21 ? ( $dt->mon + 1 ) % 12 : $dt->mon ),
    year =>
      ( ( $dt->mon == 12 and $dt->mday >= 21 ) ? $dt->year + 1 : $dt->
+year ),
    day => 21, 
    hms => '12:00:00'
};

$next->{'dt'} = DateTime->new(
    year  => $next->{'year'},
    month => $next->{'mon'},
    day   => $next->{'day'},
    hour  => substr( $next->{'hms'}, 0, 2 ) 
);

my $t = localtime;

print Time::Seconds->new(
    Time::Piece->strptime(
        sprintf( "%s-%s-%sT%s",
            $next->{'year'}, $next->{'mon'}, $next->{'day'}, $next->{'
+hms'} ),
        "%Y-%m-%dT%T"
      ) - $t
)->seconds, "\n";

print $next->{'dt'}->subtract_datetime_absolute($dt)->seconds, "\n";

# Output :
#  1308641
#  1308641
[download]

Originally posted as a Categorized Answer.