 t0 = difftime("0000-00-00 00:00:00", "0000-00-00 00:00:00", units = 'mins') t0 = difftime("0000-00-00 00:00:00", "0000-00-00 00:00:00", units = 'mins') t0 = difftime("0000-00-00 00:00:00", "0000-00-00 00:00:00", units = 'mins') t0 = difftime(