For example, for February 10, 2009 UTC at 14:25:10.1234 one would enter
2009 2 10 14 25 10.1234 in the input box fields.
(Note that a number with a decimal point must be entered into the seconds field.)

UTC is Coordinated Universal Time, which is derived from atomic time. "It [UTC] is designed to follow UT1 within +/-0.9 seconds. UTC is the basis of civil time systems and is on ordinary clocks. This definition of UTC was introduced in January, 1972, as a convenient approximation of UT1 [which is a measure of the Earth's rotation]. It is sometimes called Zulu time. " [Vallado, David, A.; "Fundamentals of Astrodynamics and Applications", Second Edition, Microcosm Press, El Segundo California, 2001, page 185.]