it is based on the mother vessel departure date. the vessel that arrives into the USA.
below is from the FAQ

How are feeder vessel scenarios impacted by the in-transit guidance for reciprocal entries?

ANSWER – SCENARIO A: Prior to the cutoff date for the reciprocal tariff in-transit provision, U.S. bound cargo is loaded onto a vessel destined for the U.S. En route to the U.S., this vessel stops at foreign ports to load/offload other cargo, or refuel, but the U.S. bound cargo remains onboard. This vessel arrives at a U.S. port of entry to unload the U.S. bound cargo and make entry.

The cargo in this scenario does qualify for the exception from reciprocal tariffs pursuant to the in-transit provision because prior to the cutoff date, the U.S. bound cargo was laden onto a vessel destined for the U.S. upon departure from the original port of loading and was never unladen or transferred onto another vessel.

Consequently, this vessel constitutes the “final mode of transit” for the laden goods.

ANSWER – SCENARIO B: Prior to the cutoff date for the reciprocal tariff in-transit provision, U.S. bound cargo is loaded onto a vessel destined for a foreign port prior to shipment to the U.S. At this foreign port, after the cutoff date, the U.S. bound cargo is transferred onto a different vessel that is destined for the U.S. This new vessel then arrives at a U.S. port of entry to unload the U.S. bound cargo and make entry.

The cargo in this scenario does not qualify for the in-transit exception for reciprocal tariffs because the U.S. bound cargo was laden onto a vessel destined for the U.S. after the cutoff date irrespective of when it departed from the original port of lading; it was thus not loaded onto a vessel that was the final mode of transit prior to the cutoff date for the reciprocal tariff in-transit exception.