fix: ignore normal preferred day assignment for month-end rolling when series starts on Feb 28th or 29th

Author: andrewmurphy353

lib/src/series.dart

@@ -173,7 +173,13 @@ sealed class Series {
     };
 
     if (monthlyOrHigher && hasMonthEndDay(normalized)) {
-      preferredDay = 31; // rollMonth will cap to last day of target month
+      // Treat last day in Feb as normal month day
+      final isFeb = normalized.month == DateTime.february;
+      final daysInFeb = (isFeb && isLeapYear(normalized.year)) ? 29 : 28;
+
+      // rollMonth will cap to last day of target month, or 
+      // 28th/29th in the case of Feb start date
+      preferredDay = isFeb ? daysInFeb : 31;
     }
 
     final List<DateTime> dates = [normalized];

test/calculator_test.dart

@@ -148,4 +148,79 @@ post_date    label                            amount        capital       intere
           () => schedule.prettyPrint(convention: convention), prints(expected));
     });
   });
+
+  group('Calculator ad-hoc calculations', () {
+    test('solve payment using a different interest frequency, leap day start', () async {
+      // This test is to check correct handling of month rolling
+      // where a series starts on the last day of Feb - see series.dart
+      // lines 175-183
+      final calculator = Calculator()
+      ..add(SeriesAdvance(
+        numberOf: 1,
+        label: 'Loan',
+        amount: 10000,
+        postDateFrom: DateTime.utc(2024, 1, 31),
+      ))
+      ..add(SeriesPayment(
+        numberOf: 36,
+        label: 'Instalment',
+        amount: null,
+        isInterestCapitalised: false,
+        postDateFrom: DateTime.utc(2024, 2, 29)//leap year
+      ))
+      ..add(SeriesPayment(
+        numberOf: 12,
+        label: 'Interest',
+        amount: 0.0,
+        isInterestCapitalised: true,
+        frequency: Frequency.quarterly,
+        postDateFrom: DateTime.utc(2024, 4, 29)
+      ));
+      final convention = US30U360();
+      final startDate = DateTime.utc(2026, 1, 31);
+
+      final pmt = await calculator.solveValue(
+          convention: convention, interestRate: 0.1, startDate: startDate);
+      final irr = await calculator.solveRate(
+          convention: convention, startDate: startDate);
+      expect(pmt, closeTo(322.27, 0.01));
+      expect(irr, closeTo(0.09999676, 1e-8));
+    });
+    test('solve payment using a different interest frequency, non-leap day start', () async {
+      // This test is to check correct handling of month rolling
+      // where a series starts on the last day of Feb - see series.dart
+      // lines 175-183
+      final calculator = Calculator()
+      ..add(SeriesAdvance(
+        numberOf: 1,
+        label: 'Loan',
+        amount: 10000,
+        postDateFrom: DateTime.utc(2026, 1, 31),
+      ))
+      ..add(SeriesPayment(
+        numberOf: 36,
+        label: 'Instalment',
+        amount: null,
+        isInterestCapitalised: false,
+        postDateFrom: DateTime.utc(2026, 2, 28)
+      ))
+      ..add(SeriesPayment(
+        numberOf: 12,
+        label: 'Interest',
+        amount: 0.0,
+        isInterestCapitalised: true,
+        frequency: Frequency.quarterly,
+        postDateFrom: DateTime.utc(2026, 4, 28)
+      ));
+      final convention = US30U360();
+      final startDate = DateTime.utc(2026, 1, 31);
+
+      final pmt = await calculator.solveValue(
+          convention: convention, interestRate: 0.1, startDate: startDate);
+      final irr = await calculator.solveRate(
+          convention: convention, startDate: startDate);
+      expect(pmt, closeTo(322.27, 0.01));
+      expect(irr, closeTo(0.09999676, 1e-8));
+    });
+  });
 }