Hmm... the output of 20180205 is correct, but the output for 20180105 looks like the one you should've seen for 20170705.
Can't figure out, without further information, why it would work incorrectly for just one value.
The code extracts the month from 20180105 and since that is less than 7, it would:
- go back 1 year to 20170105
- extract the year from it, to get 2017
- add the fiscal month "07" and date "01" to the year, to get 20170701 (start fiscal yr)
- and then jump quarters from start fiscal year. so it should return, for 20180105:

Code:

$ 
$ perl get_quarter_dates.pl 20180105
2017-07-01 2017-09-30
2017-10-01 2017-12-31
$

A few questions to get more information:

1) Could you check again if that output is really for 20180105 and not 20170705 ?
2) What output do you get for 20170705 ?
3) Are there any other dates, besides 20180105, for which the output is incorrect ?
4) Could you run the program for all the dates I ran it, in my post ?
5) Did you make any changes in the Perl program I posted ?
6) What is your Perl version ? The following command shows the version:

Code:

perl -v

I tested it on my system that has Perl 5.24 on Debian 9.

Code:

$ 
$ perl -v

This is perl 5, version 24, subversion 1 (v5.24.1) built for x86_64-linux-gnu-thread-multi
(with 73 registered patches, see perl -V for more detail)

Copyright 1987-2017, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
GNU General Public License, which may be found in the Perl 5 source kit.

Complete documentation for Perl, including FAQ lists, should be found on
this system using "man perl" or "perldoc perl".  If you have access to the
Internet, point your browser at http://www.perl.org/, the Perl Home Page.

$ 
$ uname -morsv
Linux 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u3 (2017-08-06) x86_64 GNU/Linux
$ 
$ cat /etc/debian_version
9.1
$ 
$

7) What are the versions of your Perl modules Time::Piece and Time::Seconds ? The following commands show the version:

Code:

perl -MTime::Piece -le 'print $Time::Piece::VERSION'
perl -MTime::Seconds -le 'print $Time::Seconds::VERSION'

I have version 1.31 for both these modules.

Code:

$ 
$ perl -MTime::Piece -le 'print $Time::Piece::VERSION'
1.31
$ 
$ perl -MTime::Seconds -le 'print $Time::Seconds::VERSION'
1.31
$