Time & Frequency are the inverse of each other, that is, time is a defined quantity measuring the interval between some event re-occurance while frequency represents the number of other events that occur during a specified interval.
The principle SI units for time are Seconds (Abbreviated as: S, s or sec.).
The units of frequency are Hertz ( abbreviated as Hz, formerly cycles per second or 1/sec)
Measurement of time and frequency are so fundamental that they are almost taken for granted. Yet they are invaluable in every day life as well as science and technology.
How do we know that a second is a second or that the beating of a human heart is 80 times per minute?
Ponder the more difficult problem of how we know that the Moon is about 250,000 miles away when we cannot reach out with a ruler. However, we can bounce radio waves and light beams off its surface and time their round trip.
The same technique called “time-of-flight” enables us to measure many other distances from the sub-atomic to the astronomical. To do these things we need to have determined the speed of light in a vacuum.
The measurement of distance by the time-of-flight method combines the uncertainties of both the measured time and that related to our knowledge of the speed of light.
How do the two errors combine?
That’s another part of the understanding of measurements.
Standards & Calibration – for Time and Frequency Atomic Clocks and the like,
Clocks and other time pieces – The basic measurement devices,
Frequency Measurement Devices – Counting events in a time interval
Time & Frequency – Time & Frequency Measurement Links