We calculate the hydrodynamic timescales for a spherical ultrarelativistic shell that is decelerated by the ISM and discuss the possible relations between these timescales and the observed temporal structure in γ-ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities, and that precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not fine-tuned, astrophysical parameters. The difference between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts' durations.
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