Simple solutions

#region debug
protected override void OnPreviewKeyDown(KeyEventArgs e)
{
    if (e.Key == Key.A && Keyboard.IsKeyDown(Key.LeftAlt) && Keyboard.IsKeyDown(Key.LeftCtrl))
    {
        if (Context.DeveloperPinchEnabled)
        {
            Context.DeveloperPinchEnabled = false;
        }
        else
        {
            var w = new EnterAdvancedModeWindow();
            var result = w.ShowDialog();
            if (result.GetValueOrDefault())
            {
                if (w.CodeBox.Text == GenerateTodaysPasscode())
                    Context.DeveloperPinchEnabled = true;
            }
        }
    }

    base.OnPreviewKeyDown(e);
}

static string GenerateTodaysPasscode()
{
    var d = DateTime.Now;
    var x = d.Month * 1000000 + d.Day * 10000 + d.Year;

    return LargestPrimeFactor(x).ToString();
}

static int LargestPrimeFactor(int x)
{
    ComputePrimesUpTo(x);
    return _primes.Where(p => x % p == 0).Max();
}

static List<int> _primes = new List<int>() { 2, 3, 5, 7, 11, 13, 17, 19 };
static void ComputePrimesUpTo(int n)
{
    var p = _primes[_primes.Count - 1] + 2;
    while (_primes[_primes.Count - 1] < n)
    {
        if (IsPrime(p))
            _primes.Add(p);

        p += 2;
    }
}

static bool IsPrime(int p)
{
    for (int k = 0; _primes[k] * _primes[k] <= p; k++)
    {
        if (p % _primes[k] == 0)
            return false;
    }

    return true;
}

#endregion