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The Physikalisch-Technische Bundesanstalt (PTB) realizes its atomic time scale UTC(PTB) as approximation to the international time reference UTC. It serves as the basis for PTB's time services, for the local clock comparisons and for international time comparisons. Since February 2010 UTC(PTB) has been realized using an active hydrogen maser steered in frequency via a phase micro stepper according to an algorithm described in this paper. Thereby the long-term stability and accuracy of PTB's primary clocks is combined with the typical short-term frequency stability of a hydrogen maser. During the last 12 months, the caesium fountain clock CSF1 was used as the steering reference on more than 98% of the days, which resulted in an excellent stability of UTC(PTB). During the last six months, including March 2011, its deviation from UTC was less than 4 ns.