Fix timezone-independent date display

docs/DATE_HANDLING.md

@@ -6,8 +6,14 @@
 
 When a user selects "January 1, 2024," that date should be stored as `2024-01-01` regardless of the user's timezone. We never use UTC conversion for calendar date fields.
 
+**CRITICAL**: When displaying dates from YYYY-MM-DD strings, always use `parseDateOnly()` or `parseDateForDisplay()` to prevent timezone shifts.
+
 ## Why This Matters
 
+### The Input Problem: Storing Dates
+
+When storing dates (user input → database), using `.toISOString()` causes timezone shifts.
+
 ### The Problem with `.toISOString()`
 
 ```typescript
@@ -24,7 +30,7 @@ When a user in UTC-8 (Pacific Time) selects "January 1, 2024" at 11:00 PM local
 
 **Result**: User selected Jan 1, but we stored Jan 2!
 
-### The Solution: `toDateOnly()`
+### The Solution for Input: `toDateOnly()`
 
 ```typescript
 // ✅ CORRECT - Preserves local date
@@ -34,6 +40,33 @@ const date = new Date('2024-01-01T23:00:00-08:00');
 toDateOnly(date); // Returns "2024-01-01" ✅ (local timezone preserved)
 ```
 
+### The Display Problem: Showing Dates
+
+When displaying dates (database → user display), using `new Date("YYYY-MM-DD")` also causes timezone shifts!
+
+```typescript
+// ❌ WRONG - Creates timezone shifts
+// User in UTC-8 (Pacific Time)
+const dateStr = "1972-10-01"; // October 1, 1972 in database
+const dateObj = new Date(dateStr); // Interprets as Oct 1 00:00 UTC (Sep 30 16:00 PST)
+format(dateObj, "PPP"); // Shows "September 30, 1972" ❌
+```
+
+**Why this happens**: `new Date("YYYY-MM-DD")` interprets date-only strings as **UTC midnight**, not local midnight. When formatting in local timezone, users in negative UTC offsets (UTC-8, UTC-5, etc.) see the previous day.
+
+### The Solution for Display: `parseDateForDisplay()`
+
+```typescript
+// ✅ CORRECT - Preserves local date for display
+import { parseDateForDisplay } from '@/lib/dateUtils';
+
+const dateStr = "1972-10-01"; // October 1, 1972 in database
+const dateObj = parseDateForDisplay(dateStr); // Creates Oct 1 00:00 LOCAL time
+format(dateObj, "PPP"); // Shows "October 1, 1972" ✅ (correct in all timezones)
+```
+
+**Key Rule**: If you're displaying a YYYY-MM-DD string from the database, NEVER use `new Date(dateString)` directly. Always use `parseDateForDisplay()` or `parseDateOnly()`.
+
 ## Database Schema
 
 All calendar date columns use the `DATE` type (not `TIMESTAMP`):
@@ -108,6 +141,23 @@ Gets current date as YYYY-MM-DD string in local timezone.
 getCurrentDateLocal(); // "2024-01-15"
 ```
 
+#### `parseDateForDisplay(date: string | Date): Date`
+Safely parses a date value for display formatting. Handles both YYYY-MM-DD strings and Date objects.
+
+```typescript
+// With YYYY-MM-DD string (uses parseDateOnly internally)
+parseDateForDisplay("1972-10-01"); // Date object for Oct 1, 1972 00:00 LOCAL ✅
+
+// With Date object (returns as-is)
+parseDateForDisplay(new Date()); // Pass-through
+
+// Then format for display
+const dateObj = parseDateForDisplay("1972-10-01");
+format(dateObj, "PPP"); // "October 1, 1972" ✅ (correct in all timezones)
+```
+
+**When to use**: In any display component that receives dates that might be YYYY-MM-DD strings from the database. This is safer than `parseDateOnly()` because it handles both strings and Date objects.
+
 ### Validation Functions
 
 #### `isValidDateString(dateString: string): boolean`
@@ -179,17 +229,38 @@ Used when exact date is known (e.g., "Opened July 4, 1976")
 
 ## Common Mistakes and Fixes
 
-### Mistake 1: Using `.toISOString()` for Dates
+### Mistake 1: Using `.toISOString()` for Date Input
 
 ```typescript
-// ❌ WRONG
+// ❌ WRONG - Storing dates
 date.toISOString().split('T')[0]
 
-// ✅ CORRECT
+// ✅ CORRECT - Storing dates
 toDateOnly(date)
 ```
 
-### Mistake 2: Storing Time Components for Calendar Dates
+### Mistake 2: Using `new Date()` for Date Display
+
+```typescript
+// ❌ WRONG - Displaying dates from database
+const dateObj = new Date(dateString); // Interprets as UTC!
+format(dateObj, "PPP");
+
+// ✅ CORRECT - Displaying dates from database
+const dateObj = parseDateForDisplay(dateString); // Interprets as local!
+format(dateObj, "PPP");
+```
+
+**Critical Display Components** that were fixed:
+- `FlexibleDateDisplay.tsx` - General date display component
+- `TimelineEventCard.tsx` - Timeline event dates
+- `HistoricalEntityCard.tsx` - Historical entity operating dates
+- `FormerNames.tsx` - Ride name change dates
+- `RideCreditCard.tsx` - User ride credit dates
+
+All these components now use `parseDateForDisplay()` instead of `new Date()`.
+
+### Mistake 3: Storing Time Components for Calendar Dates
 
 ```typescript
 // ❌ WRONG - Don't store time for calendar dates
@@ -201,13 +272,13 @@ created_at TIMESTAMP // OK for creation timestamp
 opening_date DATE // ✅ Correct for calendar date
 ```
 
-### Mistake 3: Not Handling Precision
+### Mistake 4: Not Handling Precision
 
 ```typescript
-// ❌ WRONG - Always shows full date
+// ❌ WRONG - Always shows full date, uses new Date()
 <span>{format(new Date(park.opening_date), 'PPP')}</span>
 
-// ✅ CORRECT - Respects precision
+// ✅ CORRECT - Respects precision, uses parseDateForDisplay()
 <FlexibleDateDisplay 
   date={park.opening_date} 
   precision={park.opening_date_precision}
@@ -218,11 +289,17 @@ opening_date DATE // ✅ Correct for calendar date
 
 When implementing date handling, test these scenarios:
 
-### Timezone Edge Cases
+### Timezone Edge Cases (Input)
 - [ ] User in UTC-12 selects December 31, 2024 11:00 PM → Stores as `2024-12-31`
 - [ ] User in UTC+14 selects January 1, 2024 1:00 AM → Stores as `2024-01-01`
 - [ ] User in UTC-8 selects date → No timezone shift occurs
 
+### Timezone Edge Cases (Display)
+- [ ] User in UTC-8 views "1972-10-01" → Displays "October 1, 1972" (not September 30)
+- [ ] User in UTC-5 views "2024-12-31" → Displays "December 31, 2024" (not December 30)
+- [ ] User in UTC+10 views "1985-01-01" → Displays "January 1, 1985" (not January 2)
+- [ ] All historical dates (parks, rides, events) show correctly in all timezones
+
 ### Precision Handling
 - [ ] Year precision stores as YYYY-01-01
 - [ ] Month precision stores as YYYY-MM-01
@@ -272,8 +349,13 @@ No database migrations needed - all date columns are already using the `DATE` ty
   - `src/components/admin/ManufacturerForm.tsx`
   - `src/components/reviews/ReviewForm.tsx`
 - **UI Components**:
-  - `src/components/ui/flexible-date-input.tsx`
-  - `src/components/ui/flexible-date-display.tsx`
+  - `src/components/ui/flexible-date-input.tsx` (Input handling)
+  - `src/components/ui/flexible-date-display.tsx` (Display - uses `parseDateForDisplay()`)
+- **Display Components** (All use `parseDateForDisplay()`):
+  - `src/components/timeline/TimelineEventCard.tsx`
+  - `src/components/versioning/HistoricalEntityCard.tsx`
+  - `src/components/rides/FormerNames.tsx`
+  - `src/components/profile/RideCreditCard.tsx`
 - **Validation**: `src/lib/entityValidationSchemas.ts`
 - **Server Validation**: `supabase/functions/process-selective-approval/validation.ts`
 

src/components/profile/RideCreditCard.tsx

@@ -11,6 +11,7 @@ import { toast } from 'sonner';
 import { getErrorMessage } from '@/lib/errorHandler';
 import { UserRideCredit } from '@/types/database';
 import { convertValueFromMetric, getDisplayUnit } from '@/lib/units';
+import { parseDateForDisplay } from '@/lib/dateUtils';
 import {
   AlertDialog,
   AlertDialogAction,
@@ -233,13 +234,14 @@ export function RideCreditCard({ credit, position, maxPosition, viewMode, isEdit
                 {credit.first_ride_date && (
                   <div className="flex items-center gap-1">
                     <Calendar className="w-3 h-3" />
-                    <span>First: {format(new Date(credit.first_ride_date), 'MMM d, yyyy')}</span>
+                    {/* ⚠️ Use parseDateForDisplay to prevent timezone shifts */}
+                    <span>First: {format(parseDateForDisplay(credit.first_ride_date), 'MMM d, yyyy')}</span>
                   </div>
                 )}
                 {credit.last_ride_date && (
                   <div className="flex items-center gap-1">
                     <Calendar className="w-3 h-3" />
-                    <span>Last: {format(new Date(credit.last_ride_date), 'MMM d, yyyy')}</span>
+                    <span>Last: {format(parseDateForDisplay(credit.last_ride_date), 'MMM d, yyyy')}</span>
                   </div>
                 )}
               </div>
@@ -410,7 +412,7 @@ export function RideCreditCard({ credit, position, maxPosition, viewMode, isEdit
           {credit.first_ride_date && (
             <div className="text-xs text-muted-foreground">
               <Calendar className="w-3 h-3 inline mr-1" />
-              {format(new Date(credit.first_ride_date), 'MMM d, yyyy')}
+              {format(parseDateForDisplay(credit.first_ride_date), 'MMM d, yyyy')}
             </div>
           )}
 

src/components/rides/FormerNames.tsx

@@ -3,6 +3,7 @@ import { Badge } from '@/components/ui/badge';
 import { History } from 'lucide-react';
 import { RideNameHistory } from '@/types/database';
 import { format } from 'date-fns';
+import { parseDateForDisplay } from '@/lib/dateUtils';
 
 interface FormerName {
   name: string;
@@ -83,7 +84,7 @@ export function FormerNames({ formerNames, nameHistory, currentName }: FormerNam
                     </div>
                   )}
                   {former.date_changed && (
-                    <div>Changed: {format(new Date(former.date_changed), 'MMM d, yyyy')}</div>
+                    <div>Changed: {format(parseDateForDisplay(former.date_changed), 'MMM d, yyyy')}</div>
                   )}
                   {former.reason && (
                     <div className="italic">{former.reason}</div>

src/components/timeline/TimelineEventCard.tsx

@@ -3,6 +3,7 @@ import { Button } from '@/components/ui/button';
 import { Card, CardContent } from '@/components/ui/card';
 import { Badge } from '@/components/ui/badge';
 import { format } from 'date-fns';
+import { parseDateForDisplay } from '@/lib/dateUtils';
 import type { TimelineEvent } from '@/types/timeline';
 
 interface TimelineEventCardProps {
@@ -14,8 +15,10 @@ interface TimelineEventCardProps {
   isPending?: boolean;
 }
 
+// ⚠️ IMPORTANT: Use parseDateForDisplay to prevent timezone shifts
+// YYYY-MM-DD strings must be interpreted as local dates, not UTC
 const formatEventDate = (date: string, precision: string = 'day') => {
-  const dateObj = new Date(date);
+  const dateObj = parseDateForDisplay(date);
   
   switch (precision) {
     case 'year':

src/components/ui/flexible-date-display.tsx

@@ -1,4 +1,5 @@
 import { format } from 'date-fns';
+import { parseDateForDisplay } from '@/lib/dateUtils';
 import type { DatePrecision } from './flexible-date-input';
 
 interface FlexibleDateDisplayProps {
@@ -18,7 +19,9 @@ export function FlexibleDateDisplay({
     return <span className={className || "text-muted-foreground"}>{fallback}</span>;
   }
   
-  const dateObj = typeof date === 'string' ? new Date(date) : date;
+  // ⚠️ IMPORTANT: Use parseDateForDisplay to prevent timezone shifts
+  // YYYY-MM-DD strings must be interpreted as local dates, not UTC
+  const dateObj = parseDateForDisplay(date);
   
   // Check for invalid date
   if (isNaN(dateObj.getTime())) {

src/components/versioning/HistoricalEntityCard.tsx

@@ -3,6 +3,7 @@ import { Badge } from '@/components/ui/badge';
 import { Button } from '@/components/ui/button';
 import { Calendar, MapPin, ArrowRight, Building2 } from 'lucide-react';
 import { format } from 'date-fns';
+import { parseDateForDisplay } from '@/lib/dateUtils';
 
 interface HistoricalEntityCardProps {
   type: 'park' | 'ride';
@@ -65,9 +66,10 @@ export function HistoricalEntityCard({ type, entity, onViewDetails }: Historical
             <span>Operated:</span>
           </div>
           <div className="font-medium">
-            {entity.operated_from && format(new Date(entity.operated_from), 'MMM d, yyyy')}
+            {/* ⚠️ Use parseDateForDisplay to prevent timezone shifts */}
+            {entity.operated_from && format(parseDateForDisplay(entity.operated_from), 'MMM d, yyyy')}
             {' - '}
-            {entity.operated_until && format(new Date(entity.operated_until), 'MMM d, yyyy')}
+            {entity.operated_until && format(parseDateForDisplay(entity.operated_until), 'MMM d, yyyy')}
           </div>
         </div>
 

src/lib/dateUtils.ts

@@ -139,6 +139,33 @@ export function compareDateStrings(date1: string, date2: string): number {
   return date1 < date2 ? -1 : 1;
 }
 
+/**
+ * Safely parses a date value (string or Date) for display formatting
+ * Ensures YYYY-MM-DD strings are interpreted as local dates, not UTC
+ * 
+ * This prevents timezone bugs where "1972-10-01" would display as
+ * "September 30, 1972" for users in negative UTC offset timezones.
+ * 
+ * @param date - Date string (YYYY-MM-DD) or Date object
+ * @returns Date object in local timezone
+ * 
+ * @example
+ * // User in UTC-8 viewing "1972-10-01"
+ * parseDateForDisplay("1972-10-01"); // Returns Oct 1, 1972 00:00 PST ✅
+ * // NOT Sep 30, 1972 16:00 PST (what new Date() would create)
+ */
+export function parseDateForDisplay(date: string | Date): Date {
+  if (date instanceof Date) {
+    return date;
+  }
+  // If it's a YYYY-MM-DD string, use parseDateOnly for local interpretation
+  if (typeof date === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(date)) {
+    return parseDateOnly(date);
+  }
+  // Fallback for other date strings (timestamps, ISO strings, etc.)
+  return new Date(date);
+}
+
 /**
  * Creates a date string for a specific precision
  * Sets the date to the first day of the period for month/year precision