Time zero means whichever of the following occurs first:
(1) For systems with “wake-up” air bag control systems, the time at which the occupant restraint control algorithm is activated; or
(2) For continuously running algorithms,
(i) The first point in the interval where a longitudinal cumulative delta-V of over 0.8 km/h (0.5 mph) is reached within a 20 ms time period; or
(ii) For vehicles that record “delta-V, lateral,” the first point in the interval where a lateral cumulative delta-V of over 0.8 km/h (0.5 mph) is reached within a 5 ms time period; or
(3) Deployment of a non-reversible deployable restraint.
49 CFR § 563.5
Motor vehicle safety standard definitions. Unless otherwise indicated, all terms that are used in this part and are defined in the Motor Vehicle Safety Standards, part 571 of this subchapter, are used as defined therein.