  The DUNE neutrino experiment far detector has a fiducial mass of 40 kt. The
O(1 M) readout channels are distributed over the 4 x 10kt modules and need to
be synchronized to O( 10 ns ) with a reliable, simple, affordable system. For
the majority of channels a simple DC-balanced protocol is used, with clock and
synchronization information encoded on the same fibre. The remaining channels
use "White Rabbit" (IEEE-1588). Small scale tests show a timing jitter of <100
ps. The DUNE timing system has been successfully prototyped at the ProtoDUNE-SP
detector at CERN.
