static const String _longestStreakKey = 'longest_login_streak';

static Future updateLoginStreak() async {
  final prefs = await SharedPreferences.getInstance();
  final now = DateTime.now().toUtc(); // or DateTime.now() for local
  final today = DateTime(now.year, now.month, now.day);

  final lastLoginMillis = prefs.getInt(_lastLoginKey);
  final currentStreak = prefs.getInt(_streakKey) ?? 0;
  var longestStreak = prefs.getInt(_longestStreakKey) ?? 0;

  DateTime? lastDate;
  if (lastLoginMillis != null) {
    lastDate = DateTime.fromMillisecondsSinceEpoch(lastLoginMillis, isUtc: true);
    lastDate = DateTime(lastDate.year, lastDate.month, lastDate.day);
  }

  int newStreak;
  if (lastDate == null) {
    newStreak = 1;
  } else if (today.difference(lastDate).inDays == 0) {
    newStreak = currentStreak;
  } else if (today.difference(lastDate).inDays == 1) {
    newStreak = currentStreak + 1;
  } else {
    newStreak = 1;
  }

  if (newStreak > longestStreak) {
    longestStreak = newStreak;
    await prefs.setInt(_longestStreakKey, longestStreak);
  }

  await prefs.setInt(_lastLoginKey, today.millisecondsSinceEpoch);
  await prefs.setInt(_streakKey, newStreak);

  return newStreak;
}

static Future getLongestStreak() async {
  final prefs = await SharedPreferences.getInstance();
  return prefs.getInt(_longestStreakKey) ?? 0;
}

Core Concepts
The streak logic relies on comparing the current login date with the last recorded login date. 
Same Day: If the current login is on the same calendar day as the last one, the streak counter remains unchanged.
Consecutive Day: If the current login is exactly one day after the last one, the streak counter is incremented, and the last login date is updated.
Broken Streak: If more than a day has passed since the last login, the streak is broken, and the counter is reset to 1.
First Login: If there is no previous login record, the streak is initialized to 1, and the current date is saved.

And calculate the longest day streak