[GNC] Configurable Year Start Day/Month

[flywire]

Fair enough. The quoted link is not part of a PR, it demonstrates the code, and
the thread from 2021 has extended into getting the functionality working
again following subsequent code changes released in V5.

All scheme reports in GnuCash are internals, interpreted reports, with much
of the repeatable code like the date options being modular. The Manual,
wiki, and Sample Report all refer to and use existing reports as examples.
Customising options is certainly part of customising reports and a
particular focus of the Sample Report.

https://wiki.gnucash.org/wiki/index.php?title=Custom_Reports&oldid=17184#Introduction
> Unfortunately, at the present time writing a custom report for GnuCash
requires a little bit of hacking.

"For help on writing reports... consult the mailing list
gnucash-devel at gnucash.org" didn't offer the guidance I was expecting for
functionality that was previously working.

https://lists.gnucash.org/pipermail/gnucash-user/2023-April/106563.html
John Ralls wrote:

> Nope, accounting periods didn't get fixed so everything I said in 2021
> about that is still true.
>
> >
> https://github.com/flywire/gnucash/blob/Fiscal-year/libgnucash/app-utils/date-utilities.scm#L557-L569
>