Monthly Principal and Interest amounts are wrong.

[juanjoaquino05]

I compared the outputs with other calculators and the values provided doesn't match.

input: { "principal" : 100000, "interestRate" : 6, "periods" : 12 }

output: { "monthlyPaymentAmount": 8606.64, "overPaymentAmount": 3284.81, "monthlyPayments": [ { "monthNumber": 0, "loanBalanceAmount": 100000.0, "debtPaymentAmount": 8113.49, "interestPaymentAmount": 493.15, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-07-24" }, { "monthNumber": 1, "loanBalanceAmount": 91886.51, "debtPaymentAmount": 8138.40, "interestPaymentAmount": 468.24, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-08-24" }, { "monthNumber": 2, "loanBalanceAmount": 83748.11, "debtPaymentAmount": 8179.87, "interestPaymentAmount": 426.77, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-09-24" }, { "monthNumber": 3, "loanBalanceAmount": 75568.24, "debtPaymentAmount": 8233.97, "interestPaymentAmount": 372.67, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-10-24" }, { "monthNumber": 4, "loanBalanceAmount": 67334.27, "debtPaymentAmount": 8263.51, "interestPaymentAmount": 343.13, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-11-24" }, { "monthNumber": 5, "loanBalanceAmount": 59070.76, "debtPaymentAmount": 8315.33, "interestPaymentAmount": 291.31, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2022-12-24" }, { "monthNumber": 6, "loanBalanceAmount": 50755.43, "debtPaymentAmount": 8348.00, "interestPaymentAmount": 258.64, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2023-01-24" }, { "monthNumber": 7, "loanBalanceAmount": 42407.43, "debtPaymentAmount": 8390.54, "interestPaymentAmount": 216.10, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2023-02-24" }, { "monthNumber": 8, "loanBalanceAmount": 34016.89, "debtPaymentAmount": 8450.07, "interestPaymentAmount": 156.57, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2023-03-24" }, { "monthNumber": 9, "loanBalanceAmount": 25566.82, "debtPaymentAmount": 8476.35, "interestPaymentAmount": 130.29, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2023-04-24" }, { "monthNumber": 10, "loanBalanceAmount": 17090.47, "debtPaymentAmount": 8522.36, "interestPaymentAmount": 84.28, "paymentAmount": 8606.64, "additionalPaymentAmount": 0, "paymentDate": "2023-05-24" }, {

        "monthNumber": 11,
        "loanBalanceAmount": 8568.11,
        "debtPaymentAmount": 8568.11,
        "interestPaymentAmount": 43.66,
        "paymentAmount": 8611.77,
        "additionalPaymentAmount": 0,
        "paymentDate": "2023-06-24"
    }
],
"earlyPayments": {}

}

[juanjoaquino05]

So, reviewing code i figured it out. It seens like a start date thing, when we pass start date, the values differ.

[joeyvmason]

I also have discovered the same issue. It's slightly off

[joeyvmason]

@juanjoaquino05 Can you provide some info about the date issue? I might be able to fix it and submit a PR

[juanjoaquino05]

Seens like when you enter an start date in the call the code its using another strategy and there is the interest difference. But when you don't you'll have to manually put the payment date in every month.

private BigDecimal calculateInterestAmount(Loan loan, BigDecimal currentLoanBalance, BigDecimal monthlyInterestRate, LocalDate paymentDate) { return paymentDate == null ? getInterestAmountByBalanceAndMonthlyInterestRate(currentLoanBalance, monthlyInterestRate) : getInterestAmountByBalanceRateAndDays( currentLoanBalance, loan.getRate(), paymentDate.minusMonths(1).lengthOfMonth(), paymentDate.minusMonths(1).lengthOfYear() ); }

[ArtyomPanfutov]

Well, there are different strategies for calculating the interest amount.

When you pass a first payment date, the actual/actual strategy is used to get the interest amount. Initially, I implemented it this way for my specific case. Unfortunately, this behavior is not documented and I should've made it configurable.