We equate it to the frequency and vibration of a quartz crystal for most watches. We also have the earth revolving around the sun as a 'standard' measure for time.
second (s): In the International System of Units (SI), the time interval equal to 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
Going further:- time and space are linked by relativistic means so, at speeds close to C time dilates........... (look up relativity)
In fact, your question "what is the speed of time?" is alot more complicated as time doesn't have 'speed' because speed is measured as a distance travelled over TIME!
Anyway - here are some definitions of time which may (or not) help as your question goes alot deeper than you might think.
http://searchcio-midmarket.techtarget.com/sDefinition/0,,sid183_gci552553,00.html
http://en.wikipedia.org/wiki/Time
http://www.m-w.com/dictionary/time
http://whatis.techtarget.com/definition/0,,sid9_gci524077,00.html
