All international flights to the US (except those departing from non-US airports with [US Preclearance Facilities][1]) 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.)


  [1]: https://www.cbp.gov/travel/preclearance