#!/usr/bin/perl -w
use strict;
use Time::Local;
use POSIX 'strftime';

my @now = localtime;

$now[4]++;

if ($now[4] == 12) {
  $now[4] = 0;
  $now[5]++;
}

$now[3] = 1;

my @then = localtime(timelocal(0, 0, 12, @now[3 .. 5]) - 86_400);

print strftime '%Y-%m-%d', @then;