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Preparing for the moment of action speeds up reaction time (RT) performance even if the particular response is unknown beforehand. When the preparatory interval, or foreperiod (FP), varies unpredictably between trials, responses usually become faster with increasing FP length. This variable-FP effect has been demonstrated to partly originate from trial-to-trial sequential effects of FP length, which are asymmetric as they occur mainly in short-FP but not in long-FP trials. In two experiments, we examined whether and how event-specific biases arising from previous target processing and responding affect both variable-FP and sequential FP effects. We found that trial-to-trial repetitions (vs. alternations) of imperative events produced response time benefits in short-FP but not in long-FP trials, almost eliminating the variable-FP effect, while the sequential FP effect remained intact. This asymmetric contribution to speeded performance in variable-FP settings suggests that sequential event-specific biases may be highly transient and not necessarily an integral part of the mental representations that guide time-based expectancy, or may be overridden by high levels of nonspecific preparation in long-FP trials. In conclusion, temporal preparation appears to be a nonspecific mechanism (i.e., generally not bound to particular event features) for prioritizing certain positions on the mental time line, on which event-specific short-term biases are superimposed if time-based preparation is weak.