Suppose you have to specify the moment in time when a given
event occurred, a "zero time". The record must be accurate to
the minute, and be obtainable even after thousands of years. All
the measures of time we currently have are relative to a well
defined zero, but the zero is not easy to backtrack exactly. One
possibility would be to take a sample of Carbon with a well
defined, very accurate amount of 14C, and say: the event
occurred when the 14C was x%. At any time, measuring the rate of
decay, you would know when the event occurred. This however,
requires a physical entity to measure, which may be lost.
Another way would be to give the time lapsed after a well
defined series of solar eclipses. In order to define precisely
the context, you would say a list of (say) five consecutive
eclipses and the places on Earth where they were total, and then
a time gap from the last of the set. At any time in the future,
you can backtrack the specified conditions into a celestial
mechanics program and find when the eve