There are multiple ways to approximate the age of an object and or event.

The speed of the earth's rotation is slowing at a predictable rate.

Based on observable data, we can determine the speed of the earth's rotation in previous historical time frames.

We can also use this to determine how long it has taken our sun and other planets to reach their current orbits.

Tree rings are another example of a reasonably accurate method of determining the age of a tree.

Ice cores are another example of a reasonably accurate method of determining the age of a glacier.

We can track the movement of tectonic plates and based on this data we can determine where they were in previous historical time frames.

Ideally, you would like to have a few different dating methods to corroborate the approximate age of an object and or event.