Runtime enforcement is a powerful technique to ensure that a running system respects some desired properties. Using an enforcement monitor, an (untrustworthy) input execution (in the form of a sequence of events) is modified into an output sequence that complies to a property. Runtime enforcement has been extensively studied over the last decade in the context of untimed properties. This paper introduces runtime enforcement of timed properties. We revisit the foundations of runtime enforcement when time between events matters.We show how runtime enforcers can be synthesized for any safety or co-safety timed property. Proposed runtime enforcers are time retardant: to produce an output sequence, additional delays are introduced between the events of the input sequence to correct it. Runtime enforcers have been prototyped and our simulation experiments validate their effectiveness.