  The resolution and the linearity of time measurements made with the CMS
electromagnetic calorimeter are studied with samples of data from test beam
electrons, cosmic rays, and beam-produced muons. The resulting time resolution
measured by lead tungstate crystals is better than 100 ps for energy deposits
larger than 10 GeV. Crystal-to-crystal synchronization with a precision of 500
ps is performed using muons produced with the first LHC beams in 2008.
