Question

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 event occurred.

Is there a standardized or well recognized method to do so?

Answer 1

the special theory of relativity implies that there is no unique "the timing system" in the Universe. Each inertial system, depending on its velocity, has a different answer to the question whether two events occurred simultaneously, and if they didn't, what was the delay in between them.

So to be able to quantify the delay between two events, you have to choose a reference frame. Most typically, you may associate your preferred reference frame with a particular pointlike object - such as the small golden seed that sits at the center of the Earth, or the tip of the Big Ben, or anything else. There is clearly no God-given preferred choice.

Once you have such an object, you may measure the proper time along its world line in spacetime by some accurate clocks, e.g. by atomic clocks. You may also associate time with events away from the world line of your benchmark objects - e.g. as the time when the photons that got to the event were emitted from your benchmark object. Such a definition of time is nicely well-defined but is constant along "light cones" and for practical purposes, one may make subtractions.

To summarize, there are many clocks and methods how to measure and coordinates how to describe events in spacetime but there is no privileged one. Quite on the contrary, relativity implies that the right labeling of "time of an event" depends not only on the preferred "time zero" - which is ambiguous due to the time-translational symmetry - but also because of the velocity that changes the notion of simultaneity and expands the duration of processes by the time dilation.