Here, we report on the possible achievement, in ultrafast electron diffraction and imaging, of temporal resolution of tens of femtoseconds through the use of chirped electron packets in combination with energy filtering. Space-charge forces in multi-electron packets accelerate leading electrons and retard trailing ones, thus inducing correlations of momentum and time. By resolving the diffraction images with an energy analyzer, well-defined temporal slices of the long electron packet can be selected. Numerical simulations show that conventional electron sources are sufficient to reach the 30-fs domain of resolution without electron packet compression. They also reveal the influence of packet shape, electron density and photoemission bandwidth on the achievable time resolution. (C) 2008 Elsevier B. V. All rights reserved.