The Naval Observatory in Washington DC is the official time source for the US Department of defense and for the United States.U.S. Naval Observatory Directorate of Time
The National Institute of Standards and Technology (NIST) Time and Frequency Division, part of the NIST Physical Measurement Laboratory, maintains the standard for frequency and time interval for the United States, provides official time to the United States, and carries out a broad program of research and service activities in time and frequency metrology.
NIST operates public Internet Time Servers, listed here:NIST Time Servers
Google Public NTP serves leap-smeared time to smooth leap seconds. Google implements Google Public NTP on their fleet of atomic clocks in data centers around the world.Google Public NTP
Amazon provides the Amazon Time Sync Service for users to synchronize their cloud servers using a fleet of satellite-connected atomic reference clocks in each of their regions.Amazon Time Sync Service