  We report on the development of a sapphire cryogenic microwave resonator
oscillator long-term fractional frequency stability of 2x10^-17Sqrt[\tau] for
integration times \tau>10^3 s and negative drift of about 2.2x10^-15/day. The
short-term frequency instability of the oscillator is highly reproducible and
also state-of-the-art: 5.6x10^-16 for an integration time of \tau ~ 20 s.
