The just noticeable difference (JND) for linearly increasing or decreasing successive time intervals (drift) was measured by means of an adaptive psychophysical procedure. Effects of number of intervals (Nint), direction (increasing or decreasing intervals), and inter onset interval (IOI) in a sequence were examined across 3 experiments. JND decreased as a function of Nint in a negatively exponential fashion, and was not affected by direction. JND increased as function of IOI, with discontinuities close to 1 s and 1.4 s IOI. The results are compatible with a principle for detection in which an internal periodic process, based on the mean IOI of a few initial intervals in the stimulus sequence, is compared with the last few intervals. Principles based on comparing successive intervals, the first and last interval, or on comparing the last interval with an internal periodic process with the same IOI as the first interval were not supported.