use strict;
use POSIX qw(strftime);

sub day_before {
    my ($ts) = @_;
    my $ts_now = strftime '%Y%m%d', $ts;
    $ts -= 22*60*60
        while strftime('%Y%m%d', $ts) eq $ts_now;
    return strftime '%Y%m%d', $ts;
};

print day_before(time);