Passage: On August 29, an upper-level low absorbed the remnants of Hurricane Lane to the west-northwest of Hawaii. The storm was assigned the designation 96C by the United States Naval Research Laboratory (NRL). Traversing an area with sea surface temperatures 2 °C (3.6 °F) above-normal, the system coalesced into a subtropical storm by August 31. On September 2, the system reached its peak intensity and began to display an eye. Afterward, the system gradually began to weaken, while accelerating northward into colder waters. On September 3, the system weakened below tropical depression intensity, back into an extratropical low. On September 4, the system was absorbed by a larger extratropical storm in the Bering Sea.

Question: On what date did the storm become subtropical?
August 31