In the United States the National Frequency Standard is maintained by the National Institute of Standards
and Technology (NIST). Official time is maintained by the United States Naval Observatory (USNO). Time
and frequency between the two agencies are closely coordinated; for practical purposes the time supplied
by each is equivalent.
For most purposes the most accurate time is transferred via the Global Positioning
System (GPS). With good receivers time can be received accurate to about fifty billionths of a second.
Time is coordinated internationally by the International Bureau of Weights and Measures (BIPM) in Paris,
which is responsible for Coordinated Universal Time (UTC) and the designation of Leap Seconds.