I'm pretty sure it's based on Eastern. Here is an excerpt from the Rules for the $5 Amazon card on December 21:
"3. Timing: The Instant Win Game begins on December 21, 2019 at 9:00 a.m. Eastern Time ("ET") and ends on December 22, 2019 at 8:59 a.m. ET (the "Promotion Period"). Administrator’s computer is the official time-keeping device for the Instant Win Game."
Each time I've taken the time to read any rules it listed the same boilerplate statement.