The most accurate atomic clocks are moderated by precise astronomical measurements and the insertion and removal of leap seconds at the very end of June and December.
Since 1967, the International System of Units (SI) has defined the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see Cesium standard). Other physical quantities, like the volt and meter, rely on the definition of the second as part of their own definitions.
A microwave radio transmitter fills the chamber with a standing wave of radio waves. The cesium atoms absorb the radio waves and emit light. The radio waves make the electrons move farther from their nuclei. When the electrons are attracted back closer by the opposite charge of the nucleus, the electrons wiggle before they settle down in their new location. This moving charge causes the light, which is a wave of alternating electricity and magnetism.
A photocell looks at the light. When the light gets dimmer, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter. This adjustment process is where most of the work and complexity of the clock lies. When a clock is first turned on, it takes a while for it to settle down before it can be trusted.
A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of course, the real clock is the original counter.
The variety of frequencies helps reception no matter what the ionospheric weather. A binary coded decimal transmission is made once per second, and on the shortwave stations, a computerized voice announcement is made every ten seconds. The radio frequencies are set by the clocks and are a precision standard, useful for adjusting receivers. The shortwave broadcast information also includes standard time intervals, UT1 time corrections, geophysical alerts (e.g. tsunami warnings), marine storm warnings, and Global Positioning System (GPS) status reports.
One can obtain a close approximation of UTC from using a GPS receiver with suitable timecode output. GPS satellites contain atomic clocks and broadcast a precise time signal based on these clocks, with time corrections to UTC updated from the ground component of the GPS system.
GPS timestamp signals can have a hardware-based precision measured in tens of nanoseconds, given a suitably equipped GPS receiver.
If you need to rate a clock by distance: WWVB and WWV are located in Fort Collins, Colorado, about 100 kilometers north of Denver at approximately 40°40'49"Nx105°02'27"W. (The antennas are all at slightly different locations.) WWV's announcements are in a male voice.
WWVH's time annoucements have a female voice. WWVH is located on Kauai Hawaii, near Kekaha, at about 21°59'16"Nx159°45'50"W.
Mainflingen's DCF77 is at about 50°01'Nx9°00'E.