The systematics of tritium (3H), 3He and 3H-3He dating are investigated for oceanic mixing systems responding to the North Atlantic surface water tritium transient. Although the 3H-3He age is a single valued function of the "true" mixing age, verbatim acceptance of the 3H-3He age will result in a substantial underestimate of the mixing age for systems with timescales approaching the time elapsed since the tritium transient (1964-1965) or greater....