fix: handle datetime-local timezone conversion correctly

- Convert datetime-local values to UTC ISO strings before sending to API
  (new Date(r).toISOString()) in both create and update flows
- Parse UTC ISO strings directly in API instead of manual Date.UTC() parsing
  that treated local components as UTC, causing timezone offset bugs
- Convert UTC API response times to local on load using new Date(str + 'Z')
  and date-fns format() to avoid double-timezone shift when editing

src/app/api/tasks/[id]/route.ts

@@ -77,15 +77,9 @@ export async function PUT(
         nextOccurrence: nextOccurrence ? new Date(nextOccurrence) : undefined,
         reminders: {
           deleteMany: {},
-          create: (reminders || []).map((r: string) => {
-            // Parse local datetime string and construct Date to preserve local time
-            const [datePart, timePart] = r.split('T');
-            const [year, month, day] = datePart.split('-').map(Number);
-            const [hours, minutes] = timePart.split(':').map(Number);
-            return {
-              reminder: new Date(Date.UTC(year, month - 1, day, hours, minutes)),
-            };
-          }),
+          create: (reminders || []).map((r: string) => ({
+            reminder: new Date(r),
+          })),
         },
       },
       include: {

src/app/api/tasks/route.ts

@@ -93,15 +93,9 @@ export async function POST(request: NextRequest) {
         parentTaskId: parentTaskId || undefined,
         sortOrder: (maxSort._max.sortOrder ?? 0) + 1,
         reminders: {
-          create: (reminders || []).map((r: string) => {
-            // Parse local datetime string and construct Date to preserve local time
-            const [datePart, timePart] = r.split('T');
-            const [year, month, day] = datePart.split('-').map(Number);
-            const [hours, minutes] = timePart.split(':').map(Number);
-            return {
-              reminder: new Date(Date.UTC(year, month - 1, day, hours, minutes)),
-            };
-          }),
+          create: (reminders || []).map((r: string) => ({
+            reminder: new Date(r),
+          })),
         },
       },
       include: {

src/components/EditTaskModal.tsx

@@ -44,7 +44,11 @@ export default function EditTaskModal() {
         .then(data => {
           setSubtasks(data.subtasks || []);
           if (data.reminders && data.reminders.length > 0) {
-            setReminders(data.reminders.map((r: any) => r.reminder));
+            // API returns UTC times — convert to local for datetime-local input
+            setReminders(data.reminders.map((r: any) => {
+              const d = new Date(r.reminder + 'Z');
+              return format(d, "yyyy-MM-dd'T'HH:mm");
+            }));
           }
         })
         .catch(console.error);
@@ -81,7 +85,9 @@ export default function EditTaskModal() {
           status,
           recurrenceRule,
           recurrenceInterval,
-          reminders: reminders.filter((r) => r !== ''),
+          reminders: reminders
+            .filter((r) => r !== '')
+            .map((r) => new Date(r).toISOString()),
         }),
       });
 

src/components/NewTaskModal.tsx

@@ -66,7 +66,9 @@ export default function NewTaskModal() {
           status,
           recurrenceRule,
           recurrenceInterval,
-          reminders: reminders.filter((r) => r !== ''),
+          reminders: reminders
+            .filter((r) => r !== '')
+            .map((r) => new Date(r).toISOString()),
         }),
       });