Fix timezone-independent date display

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