All international flights to the US (except those departing from non-US airports with US Preclearance Facilities) clear US Immigration and Customs when first arriving on US soil. Your departure airport HKG does not have a US Preclearance Facility.
Thus, you will have "returned to the United States" at LAX on December 29, 2017.
(Note that this answer does not address a traveler's "arrival date in the US" when the traveler passes US Immigration and Customs overseas in a US Preclearance Facility, and physically arrives in the US on the next calendar day. I don't know the answer to that question.)