when did united states start using universal time code example

Example: utc full form in time

Universal Time Coordinated: Prior to 1972, this time was called 
Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time 
or Universal Time Coordinated (UTC). It is a coordinated time scale, 
maintained by the Bureau International des Poids et Mesures (BIPM). 
It is also known as "Z time" or "Zulu Time".