The present study investigated the effect of an imminent fearful stimulus on an ongoing temporal task. Participants judged the duration of a blank temporal interval followed by a fearful or a neutral image. Results showed an underestimation of the duration in the fearful condition relative to the neutral condition, but only when the occurrence of the fearful image was difficult to predict. ERPs results for the blank temporal interval found no effect of the fearful stimulus on the contingent negative variation (CNV) amplitude in the clock stage. However, after the image onset, there was a larger P1 for the fearful relative to the neutral condition. Although this effect was indistinguishable regardless of whether the fearful event could be easily predicted, a late positive potential (LPP) component displayed larger amplitude only for unpredictable fearful stimuli. The time-frequency results showed enhanced delta-theta power (0.5-7.5 Hz) for the unpredictable fearful stimuli in the late stage. Importantly, the enhanced delta-theta rhythm correlated negatively with the duration judgments. Together, these results suggest that an unpredictable fearful event might divert more attention away from the counting process in the working memory stage, resulting in missing ticks and temporal underestimation.