100-ns-level timing holdover after 12 years for rubidium atomic fountains

While atomic frequency standards are improving at a staggering pace, the timing community has relied on the same continuously running atomic clocks for decades: commercial cesium beams and hydrogen masers. Challenges in incorporating the latest technological advancements into operational clocks has resulted in technology lag compared with frequency standards that consequently impacts timing applications, such as system synchronization, positioning and timescales. The first cold-atom clocks to contribute to the free running international atomic timescale, EAL, are the four rubidium fountains in operation at the U.S.~Naval Observatory in Washington, DC, that came online in 2011. With 12 years of uninterrupted data from the International Bureau of Weights and Measures (BIPM) from Modified Julian Date (MJD) 56074 to MJD 60429, we report on the long-term timing performance of these clocks. The highest performing fountain exhibits TDEV of 8~ns at $\sim 3$~years and a holdover of BIPM's best timescale of $\pm14$~ns at 12 years.