Interference effect
Timing theorists asume that prospective timing is functionally equivalent to many other perceptual and cognitive tasks in that it is a deliberate, controlled process that requires attentional resources... In this view, concurrent temporal and nontemporal tasks compete for limited resources, with the result that less attention is devoted to timekeeping than otherwise would be the case.  The fewer resources devoted to time, the more inaccurate and variable the time judgements.