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We present the design of the Tempo mote that operates on ambient energy harvested from the environment. Equipped with a ultra low-power timing module that acquires Coordinated Universal Time (UTC) information from a longrange radio transmitter, Tempo can persistently access the global time and is thereby resilient to power outages. A prototype implementation of the Tempo mote shows that it achieves millisecond level accuracy at 100 μA current draw. We argue that one can more generally leverage the access to global time across all nodes of a wireless sensor network to overcome the challenges related to the very tight energy budget that is inherent with harvesting ambient energy.