My waiver will expire on the 31 December 2014. Do I need a waiver all the time I am in the USA? I plan to enter the USA on the 30 December and leave on the 16 January.
I assume you're referring to your ESTA travel authorisation - if so: Direct quote from the ESTA Q&A page:
My travel authorization is valid for travel to the United States but will expire before I depart. Do I need to apply for a new one before my trip?
No. ESTA travel authorization needs only to be valid upon arrival in the United States.
Thus you should be fine - but do remember the immigration officers have the last say and may not necessarily like the fact you're entering the day before the ESTA expires.