Bug Report and Analysis: Loan Repayment Schedule Shift in GnuCash

[Genieliu]

Dear GnuCash Community,

I am a newcomer to GnuCash and have been exploring its features. While 
using the loan repayment tool to generate scheduled transactions, I 
encountered an issue with the loan review functionality, where the 
repayment schedule appears to be misaligned. This problem has been 
previously discussed on Reddit 
(https://www.reddit.com/r/GnuCash/comments/umzb91/mortgage_amortization/) 
and a bug report has been filed on Bugzilla 
(https://bugs.gnucash.org/show_bug.cgi?id=797824).

After investigating the issue, I believe I have identified the root 
cause. As I am unfamiliar with the contribution process for GNU 
projects, I thought this would be the appropriate channel to share my 
findings.

Steps to Reproduce the Issue:

1. Navigate to Actions -> Scheduled Transactions -> Mortgage & Loan 
Repayments.

2. Loan Details: Amount 1000, Start Date is 02/01/2025, Length: 6 Months 
Remaining: 6

3. Loan Repayment: Monthly Start Date is 02/01/2025, Every 1 Month on 
the 1st, weekends: no change

4. Loan Review: when selecting the current year, we got the schedules 
starting from 01/31/2025, 02/01/2025 ... 06/01/2025. While if we choose 
whole loan, the 01/31/2025 is missing and the last repayment is still 
06/01/2025

The expected repayment schedule should be 02/01/2025, 03/01/2025 ... 
07/01/2025. It seems the first repayment is incorrectly set to one day 
before the start date, causing the entire schedule to shift.

I think the issue originates in the loan_rev_recalc_schedule function 
within the assistant-loan.cpp file, 
https://github.com/Gnucash/gnucash/blob/f6c73b15fb6896b91bbbf1845cee41d7077ee5b3/gnucash/gnome/assistant-loan.cpp#L2062

In this function, curDate is initialized as the start date minus one 
day, then we got the nextDate.

Subsequently, the for loop uses curDate as the first repayment date, 
resulting in the first repayment being scheduled one day prior to the 
intended start date. This misalignment propagates through the entire 
repayment schedule, shifting the final repayment date as well.

curDate = start;

g_date_subtract_days( &curDate, 1 );

recurrenceListNextInstance(ldd->ld.repayment_schedule, &curDate, &nextDate);

I hope my analysis helps in confirming the root cause of this issue. I 
would be more than happy to submit a pull request to address the 
problem. Please let me know if additional details or further 
clarification are required to proceed.


Jianshi Liu
Senior Engineer - Machine Learning
Email: genie_liu at outlook.com



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