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A swimming race illustrates the simple principles involved in measuring time.
You can only establish the time for the race if it was timed by two or more qualified eyewitnesses who observed the start, the progress and the finish.
Greenland boasts the Isua Supracrustal rocks (3.7 to 3.8 billion years old), while rocks in Swaziland are 3.4 to 3.5 billion years.
Samples in Western Australia run 3.4 to 3.6 billion years old.
We note that at the instant the swimmer touches the edge of the pool our wristwatch reads and 53 seconds.
How long has the competitor taken to swim the 1,500 metre race?