Application of geologic ages expressed in numeric time (millions of years) is a recent but rapidly expanding factor in petroleum exploration (rates of tectonic movements; basin models; geohistory analysis; prediction of reservoir quality, source potential, and HC maturation; correlation of geophysics with geologic events, etc.). This paper proposes an emendation of published versions of the Jurassic time scale. The standard geochronologic subdivision is combined with a linear-numeric scale, with a biostratigraphic scheme (ammonites, calcareous nannofossils), and with the geomagnetic-reversal time scale. To this purpose, basement ages at some Deep Sea Drilling Project sites are evaluated and related to the geomagnetic-reversal scale. The numeric scale certainly cannot be regarded as precisely and firmly determined throughout the Jurassic, but it seems adequate as a broad framework.