Consider current month in FIRE calculation (#1349)

libs/ui/src/lib/fire-calculator/fire-calculator.component.ts

@@ -25,6 +25,7 @@ import {
   Tooltip
 } from 'chart.js';
 import * as Color from 'color';
+import { getMonth } from 'date-fns';
 import { isNumber } from 'lodash';
 import { Subject, takeUntil } from 'rxjs';
 
@@ -50,6 +51,8 @@ export class FireCalculatorComponent
 
   @ViewChild('chartCanvas') chartCanvas;
 
+  private readonly CONTRIBUTION_PERIOD = 12;
+
   public calculatorForm = this.formBuilder.group({
     annualInterestRate: new FormControl<number>(undefined),
     paymentPerPeriod: new FormControl<number>(undefined),
@@ -296,11 +299,15 @@ export class FireCalculatorComponent
       label: $localize`Savings`
     };
 
+    const monthsPassedInCurrentYear = getMonth(new Date());
+
     for (let period = 1; period <= t; period++) {
+      const periodInMonths =
+        period * this.CONTRIBUTION_PERIOD - monthsPassedInCurrentYear;
       const { interest, principal, totalAmount } =
         this.fireCalculatorService.calculateCompoundInterest({
           P,
-          period,
+          periodInMonths,
           PMT,
           r
         });

libs/ui/src/lib/fire-calculator/fire-calculator.service.ts

@@ -4,35 +4,32 @@ import Big from 'big.js';
 @Injectable()
 export class FireCalculatorService {
   private readonly COMPOUND_PERIOD = 12;
-  private readonly CONTRIBUTION_PERIOD = 12;
 
   public constructor() {}
 
   public calculateCompoundInterest({
     P,
-    period,
+    periodInMonths,
     PMT,
     r
   }: {
     P: number;
-    period: number;
+    periodInMonths: number;
     PMT: number;
     r: number;
   }) {
     let interest = new Big(0);
-    const principal = new Big(P).plus(
+    const principal = new Big(P).plus(new Big(PMT).mul(periodInMonths));
-      new Big(PMT).mul(this.CONTRIBUTION_PERIOD).mul(period)
-    );
     let totalAmount = principal;
 
     if (r) {
       const compoundInterestForPrincipal = new Big(1)
         .plus(new Big(r).div(this.COMPOUND_PERIOD))
-        .pow(new Big(this.COMPOUND_PERIOD).mul(period).toNumber());
+        .pow(periodInMonths);
       const compoundInterest = new Big(P).mul(compoundInterestForPrincipal);
       const contributionInterest = new Big(
         new Big(PMT).mul(compoundInterestForPrincipal.minus(1))
-      ).div(new Big(r).div(this.CONTRIBUTION_PERIOD));
+      ).div(new Big(r).div(this.COMPOUND_PERIOD));
       interest = compoundInterest.plus(contributionInterest).minus(principal);
       totalAmount = compoundInterest.plus(contributionInterest);
     }