fix: Use user's local timezone for streak date calculations

src/lib/dates/streakDateUtils.ts

@@ -0,0 +1,90 @@
+/**
+ * Timezone-Aware Streak Date Utilities
+ *
+ * Prevents streak miscalculation for users in non-UTC timezones.
+ * All streak comparisons use the user's local date, not UTC.
+ */
+
+/**
+ * Get the current local date string (YYYY-MM-DD) for a given timezone.
+ *
+ * @param timezone - IANA timezone string (e.g., "America/New_York")
+ * @returns Date string in YYYY-MM-DD format
+ */
+export function getLocalDateString(timezone: string = "UTC"): string {
+  try {
+    return new Intl.DateTimeFormat("en-CA", {
+      timeZone: timezone,
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+    }).format(new Date());
+  } catch {
+    // Fallback to UTC if timezone is invalid
+    return new Date().toISOString().slice(0, 10);
+  }
+}
+
+/**
+ * Convert a UTC timestamp to a local date string for a given timezone.
+ */
+export function utcToLocalDate(utcTimestamp: string | Date, timezone: string = "UTC"): string {
+  const date = typeof utcTimestamp === "string" ? new Date(utcTimestamp) : utcTimestamp;
+  try {
+    return new Intl.DateTimeFormat("en-CA", {
+      timeZone: timezone,
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+    }).format(date);
+  } catch {
+    return date.toISOString().slice(0, 10);
+  }
+}
+
+/**
+ * Check if two dates are consecutive days in a given timezone.
+ */
+export function areConsecutiveDays(
+  date1: string,
+  date2: string,
+  timezone: string = "UTC"
+): boolean {
+  const d1 = new Date(date1 + "T12:00:00Z");
+  const d2 = new Date(date2 + "T12:00:00Z");
+  const diffMs = Math.abs(d2.getTime() - d1.getTime());
+  const diffDays = Math.round(diffMs / (1000 * 60 * 60 * 24));
+  return diffDays === 1;
+}
+
+/**
+ * Calculate streak from a sorted list of contribution dates.
+ *
+ * @param dates - Array of date strings (YYYY-MM-DD), most recent first
+ * @param userTimezone - User's IANA timezone string
+ * @returns Current streak length in days
+ */
+export function calculateStreak(dates: string[], userTimezone: string = "UTC"): number {
+  if (!dates || dates.length === 0) return 0;
+
+  const today = getLocalDateString(userTimezone);
+  const yesterday = getLocalDateString(userTimezone);
+
+  const uniqueDates = [...new Set(dates)].sort().reverse();
+
+  // Streak must start from today or yesterday
+  if (uniqueDates[0] !== today && uniqueDates[0] !== yesterday) {
+    return 0;
+  }
+
+  let streak = 1;
+  for (let i = 1; i < uniqueDates.length; i++) {
+    if (areConsecutiveDays(uniqueDates[i], uniqueDates[i - 1], userTimezone)) {
+      streak++;
+    } else {
+      break;
+    }
+  }
+
+  return streak;
+}