Question

What types of phenomenon can be used as a time standard?

Answer 1

Time standard is the specification  of measuring the time or rate at which time passes.
One example of time standard is based on rotation of the earth.
Earth rotates around the sun and we can find its time period by  equating the gravitational force between earth & sun and centripetal force between them

Answer 2

Hard question as general quantum theory has time being a malleable substance. For instance, if you fly close to a black hole or at massive speeds (just under the speed of light, for example) then you will experience physiological time distortion. What feels like a week to you might be a century to those people living on Earth. This results in the conclusion that there can be no true time standard ANYWHERE.

That said, in practical terms, anything that has an extremely predictable progression can be a time standard. These include radioactive decay, planetary orbit, speed of light, ect.

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