A novel time digitizer has been commissioned to facilitate the measurement of isometric lifetimes or the measurement of the lifetimes of nuclear reaction products from μs upwards. The time digitizer has been used in conjunction with a beam chopper on the Liverpool University EN tandem Van de Graaff accelerator to measure and store the time intervals between the beam off markers and every subsequent γ-ray occuring in the beam off period. The device has been interfaced as a pseudo ADC into the Liverpool on-line computer system such that the beam on and off periods and time dispersion can be set from the on-line display and are then used for spectrum labelling. Exponential decay spectra of 1000 channels or more may be quickly built up on magnetic tape for every γ-ray contained in a Ge(Li) spectrum and the decay of selected γ-rays or sections of the spectrum may be monitored via the on-line display. The system has been tested in the range 20 μs to 20 s against accurately measured isometric half-lives in 38K, 24Na and 77Se. Further measurements have been carried out for several other cases where there has been disagreement in the literature.