From d1296f1a90f38dae8569a571823f003c2cd6c324 Mon Sep 17 00:00:00 2001
From: Chris <chrisvel@users.noreply.github.com>
Date: Sun, 28 Dec 2025 17:08:33 +0200
Subject: [PATCH] Fix skipping month (#745)

---
 backend/services/recurringTaskService.js      | 25 +++++++++++++---
 .../tests/integration/recurring-tasks.test.js | 30 +++++++++++++++++++
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/backend/services/recurringTaskService.js b/backend/services/recurringTaskService.js
index a31c209..1203f69 100644
--- a/backend/services/recurringTaskService.js
+++ b/backend/services/recurringTaskService.js
@@ -145,12 +145,29 @@ const calculateMonthlyWeekdayRecurrence = (
 };
 
 const calculateMonthlyLastDayRecurrence = (fromDate, interval) => {
-    const nextDate = new Date(fromDate);
-    nextDate.setUTCMonth(nextDate.getUTCMonth() + interval);
+    // Calculate target year and month directly to avoid date overflow
+    // (e.g., Jan 31 + 1 month via setUTCMonth would overflow to March)
+    const currentMonth = fromDate.getUTCMonth();
+    const currentYear = fromDate.getUTCFullYear();
 
-    nextDate.setUTCMonth(nextDate.getUTCMonth() + 1, 0);
+    const totalMonths = currentMonth + interval;
+    const targetYear = currentYear + Math.floor(totalMonths / 12);
+    const targetMonth = totalMonths % 12;
 
-    return nextDate;
+    // Get last day of target month by creating date at day 0 of following month
+    const lastDayOfMonth = new Date(
+        Date.UTC(
+            targetYear,
+            targetMonth + 1, // next month
+            0, // day 0 = last day of previous month
+            fromDate.getUTCHours(),
+            fromDate.getUTCMinutes(),
+            fromDate.getUTCSeconds(),
+            fromDate.getUTCMilliseconds()
+        )
+    );
+
+    return lastDayOfMonth;
 };
 
 const getFirstWeekdayOfMonth = (year, month, weekday) => {
diff --git a/backend/tests/integration/recurring-tasks.test.js b/backend/tests/integration/recurring-tasks.test.js
index 2ae1e02..76a3f84 100644
--- a/backend/tests/integration/recurring-tasks.test.js
+++ b/backend/tests/integration/recurring-tasks.test.js
@@ -303,6 +303,36 @@ describe('Recurring Tasks', () => {
                 expect(nextDate.getUTCDate()).toBe(expectedDate.getUTCDate());
             });
 
+            it('should not skip months for monthly_last_day when starting from 31st', async () => {
+                // Bug fix: Jan 31 -> should go to Feb 28, not March 31
+                const jan31 = new Date(Date.UTC(2025, 0, 31, 0, 0, 0, 0));
+
+                const taskData = {
+                    name: 'End of Month Task',
+                    recurrence_type: 'monthly_last_day',
+                    recurrence_interval: 1,
+                    due_date: jan31.toISOString().split('T')[0],
+                };
+
+                const response = await agent.post('/api/task').send(taskData);
+                const task = await Task.findByPk(response.body.id);
+
+                // First occurrence: Jan 31 -> Feb 28
+                const nextDate1 = calculateNextDueDate(task, jan31);
+                expect(nextDate1.getUTCMonth()).toBe(1); // February
+                expect(nextDate1.getUTCDate()).toBe(28);
+
+                // Second occurrence: Feb 28 -> Mar 31
+                const nextDate2 = calculateNextDueDate(task, nextDate1);
+                expect(nextDate2.getUTCMonth()).toBe(2); // March
+                expect(nextDate2.getUTCDate()).toBe(31);
+
+                // Third occurrence: Mar 31 -> Apr 30
+                const nextDate3 = calculateNextDueDate(task, nextDate2);
+                expect(nextDate3.getUTCMonth()).toBe(3); // April
+                expect(nextDate3.getUTCDate()).toBe(30);
+            });
+
             it('should handle monthly recurrence when day does not exist in target month', async () => {
                 // Create a task for Jan 31
                 const jan31 = new Date(Date.UTC(2024, 0, 31, 0, 0, 0, 0));