  For a simple electromagnetic model of a clock introduced by Jefimenko (clock
$\#$ 1 in 1996 {\it Am. J. Phys.} {\bf 64} 812), a change of the rate of the
clock when it is set in uniform motion is calculated exactly, employing the
correct equation of motion of a charged particle in the electromagnetic field
and the universal boostability assumption. Thus, for the clock under
consideration, a dynamical content of the clock retardation is demonstrated.
Somewhat surprisingly, the analysis presented discloses that some familiar
relativistic generalisations concerning the retardation of clocks have to be
amended.
