We give a coalgebraic formulation of timed processes and their operational semantics. We model time by a monoid called a "time domain", and we model processes by "timed transition systems", which amount to partial monoid actions of the time domain or, equivalently, coalgebras for an "evolution comonad" generated by the time domain. All our examples of time domains satisfy a partial closure property, yielding a distributive law of a monad for total monoid actions over the evolution comonad, and hence a distributive law of the evolution comonad over a dual comonad for total monoid actions. We show that the induced coalgebras are exactly timed transition systems with delay operators. We then integrate our coalgebraic formulation of time qua timed transition systems into Turi and Plotkin's formulation of structural operational semantics in terms of distributive laws. We combine timing with action via the more general study of the combination of two arbit...